FreeBSD/Linux Kernel Cross Reference
sys/kern/sched_ule.c

     /*-
      * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
      *
      * Copyright (c) 2002-2007, Jeffrey Roberson <jeff@freebsd.org>
      * All rights reserved.
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice unmodified, this list of conditions, and the following
     *    disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
     * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
     * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
     * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
     * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
     * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
     * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
     * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
     * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     */
    
    /*
     * This file implements the ULE scheduler.  ULE supports independent CPU
     * run queues and fine grain locking.  It has superior interactive
     * performance under load even on uni-processor systems.
     *
     * etymology:
     *   ULE is the last three letters in schedule.  It owes its name to a
     * generic user created for a scheduling system by Paul Mikesell at
     * Isilon Systems and a general lack of creativity on the part of the author.
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include "opt_hwpmc_hooks.h"
    #include "opt_sched.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kdb.h>
    #include <sys/kernel.h>
    #include <sys/ktr.h>
    #include <sys/limits.h>
    #include <sys/lock.h>
    #include <sys/mutex.h>
    #include <sys/proc.h>
    #include <sys/resource.h>
    #include <sys/resourcevar.h>
    #include <sys/sched.h>
    #include <sys/sdt.h>
    #include <sys/smp.h>
    #include <sys/sx.h>
    #include <sys/sysctl.h>
    #include <sys/sysproto.h>
    #include <sys/turnstile.h>
    #include <sys/umtxvar.h>
    #include <sys/vmmeter.h>
    #include <sys/cpuset.h>
    #include <sys/sbuf.h>
    
    #ifdef HWPMC_HOOKS
    #include <sys/pmckern.h>
    #endif
    
    #ifdef KDTRACE_HOOKS
    #include <sys/dtrace_bsd.h>
    int __read_mostly               dtrace_vtime_active;
    dtrace_vtime_switch_func_t      dtrace_vtime_switch_func;
    #endif
    
    #include <machine/cpu.h>
    #include <machine/smp.h>
    
    #define KTR_ULE 0
    
    #define TS_NAME_LEN (MAXCOMLEN + sizeof(" td ") + sizeof(__XSTRING(UINT_MAX)))
    #define TDQ_NAME_LEN    (sizeof("sched lock ") + sizeof(__XSTRING(MAXCPU)))
    #define TDQ_LOADNAME_LEN        (sizeof("CPU ") + sizeof(__XSTRING(MAXCPU)) - 1 + sizeof(" load"))
    
    /*
     * Thread scheduler specific section.  All fields are protected
     * by the thread lock.
     */
    struct td_sched {       
            struct runq     *ts_runq;       /* Run-queue we're queued on. */
            short           ts_flags;       /* TSF_* flags. */
            int             ts_cpu;         /* CPU that we have affinity for. */
            int             ts_rltick;      /* Real last tick, for affinity. */
            int             ts_slice;       /* Ticks of slice remaining. */
            u_int           ts_slptime;     /* Number of ticks we vol. slept */
            u_int           ts_runtime;     /* Number of ticks we were running */
           int             ts_ltick;       /* Last tick that we were running on */
           int             ts_ftick;       /* First tick that we were running on */
           int             ts_ticks;       /* Tick count */
   #ifdef KTR
           char            ts_name[TS_NAME_LEN];
   #endif
   };
   /* flags kept in ts_flags */
   #define TSF_BOUND       0x0001          /* Thread can not migrate. */
   #define TSF_XFERABLE    0x0002          /* Thread was added as transferable. */
   
   #define THREAD_CAN_MIGRATE(td)  ((td)->td_pinned == 0)
   #define THREAD_CAN_SCHED(td, cpu)       \
       CPU_ISSET((cpu), &(td)->td_cpuset->cs_mask)
   
   _Static_assert(sizeof(struct thread) + sizeof(struct td_sched) <=
       sizeof(struct thread0_storage),
       "increase struct thread0_storage.t0st_sched size");
   
   /*
    * Priority ranges used for interactive and non-interactive timeshare
    * threads.  The timeshare priorities are split up into four ranges.
    * The first range handles interactive threads.  The last three ranges
    * (NHALF, x, and NHALF) handle non-interactive threads with the outer
    * ranges supporting nice values.
    */
   #define PRI_TIMESHARE_RANGE     (PRI_MAX_TIMESHARE - PRI_MIN_TIMESHARE + 1)
   #define PRI_INTERACT_RANGE      ((PRI_TIMESHARE_RANGE - SCHED_PRI_NRESV) / 2)
   #define PRI_BATCH_RANGE         (PRI_TIMESHARE_RANGE - PRI_INTERACT_RANGE)
   
   #define PRI_MIN_INTERACT        PRI_MIN_TIMESHARE
   #define PRI_MAX_INTERACT        (PRI_MIN_TIMESHARE + PRI_INTERACT_RANGE - 1)
   #define PRI_MIN_BATCH           (PRI_MIN_TIMESHARE + PRI_INTERACT_RANGE)
   #define PRI_MAX_BATCH           PRI_MAX_TIMESHARE
   
   /*
    * Cpu percentage computation macros and defines.
    *
    * SCHED_TICK_SECS:     Number of seconds to average the cpu usage across.
    * SCHED_TICK_TARG:     Number of hz ticks to average the cpu usage across.
    * SCHED_TICK_MAX:      Maximum number of ticks before scaling back.
    * SCHED_TICK_SHIFT:    Shift factor to avoid rounding away results.
    * SCHED_TICK_HZ:       Compute the number of hz ticks for a given ticks count.
    * SCHED_TICK_TOTAL:    Gives the amount of time we've been recording ticks.
    */
   #define SCHED_TICK_SECS         10
   #define SCHED_TICK_TARG         (hz * SCHED_TICK_SECS)
   #define SCHED_TICK_MAX          (SCHED_TICK_TARG + hz)
   #define SCHED_TICK_SHIFT        10
   #define SCHED_TICK_HZ(ts)       ((ts)->ts_ticks >> SCHED_TICK_SHIFT)
   #define SCHED_TICK_TOTAL(ts)    (max((ts)->ts_ltick - (ts)->ts_ftick, hz))
   
   /*
    * These macros determine priorities for non-interactive threads.  They are
    * assigned a priority based on their recent cpu utilization as expressed
    * by the ratio of ticks to the tick total.  NHALF priorities at the start
    * and end of the MIN to MAX timeshare range are only reachable with negative
    * or positive nice respectively.
    *
    * PRI_RANGE:   Priority range for utilization dependent priorities.
    * PRI_NRESV:   Number of nice values.
    * PRI_TICKS:   Compute a priority in PRI_RANGE from the ticks count and total.
    * PRI_NICE:    Determines the part of the priority inherited from nice.
    */
   #define SCHED_PRI_NRESV         (PRIO_MAX - PRIO_MIN)
   #define SCHED_PRI_NHALF         (SCHED_PRI_NRESV / 2)
   #define SCHED_PRI_MIN           (PRI_MIN_BATCH + SCHED_PRI_NHALF)
   #define SCHED_PRI_MAX           (PRI_MAX_BATCH - SCHED_PRI_NHALF)
   #define SCHED_PRI_RANGE         (SCHED_PRI_MAX - SCHED_PRI_MIN + 1)
   #define SCHED_PRI_TICKS(ts)                                             \
       (SCHED_TICK_HZ((ts)) /                                              \
       (roundup(SCHED_TICK_TOTAL((ts)), SCHED_PRI_RANGE) / SCHED_PRI_RANGE))
   #define SCHED_PRI_NICE(nice)    (nice)
   
   /*
    * These determine the interactivity of a process.  Interactivity differs from
    * cpu utilization in that it expresses the voluntary time slept vs time ran
    * while cpu utilization includes all time not running.  This more accurately
    * models the intent of the thread.
    *
    * SLP_RUN_MAX: Maximum amount of sleep time + run time we'll accumulate
    *              before throttling back.
    * SLP_RUN_FORK:        Maximum slp+run time to inherit at fork time.
    * INTERACT_MAX:        Maximum interactivity value.  Smaller is better.
    * INTERACT_THRESH:     Threshold for placement on the current runq.
    */
   #define SCHED_SLP_RUN_MAX       ((hz * 5) << SCHED_TICK_SHIFT)
   #define SCHED_SLP_RUN_FORK      ((hz / 2) << SCHED_TICK_SHIFT)
   #define SCHED_INTERACT_MAX      (100)
   #define SCHED_INTERACT_HALF     (SCHED_INTERACT_MAX / 2)
   #define SCHED_INTERACT_THRESH   (30)
   
   /*
    * These parameters determine the slice behavior for batch work.
    */
   #define SCHED_SLICE_DEFAULT_DIVISOR     10      /* ~94 ms, 12 stathz ticks. */
   #define SCHED_SLICE_MIN_DIVISOR         6       /* DEFAULT/MIN = ~16 ms. */
   
   /* Flags kept in td_flags. */
   #define TDF_PICKCPU     TDF_SCHED0      /* Thread should pick new CPU. */
   #define TDF_SLICEEND    TDF_SCHED2      /* Thread time slice is over. */
   
   /*
    * tickincr:            Converts a stathz tick into a hz domain scaled by
    *                      the shift factor.  Without the shift the error rate
    *                      due to rounding would be unacceptably high.
    * realstathz:          stathz is sometimes 0 and run off of hz.
    * sched_slice:         Runtime of each thread before rescheduling.
    * preempt_thresh:      Priority threshold for preemption and remote IPIs.
    */
   static u_int __read_mostly sched_interact = SCHED_INTERACT_THRESH;
   static int __read_mostly tickincr = 8 << SCHED_TICK_SHIFT;
   static int __read_mostly realstathz = 127;      /* reset during boot. */
   static int __read_mostly sched_slice = 10;      /* reset during boot. */
   static int __read_mostly sched_slice_min = 1;   /* reset during boot. */
   #ifdef PREEMPTION
   #ifdef FULL_PREEMPTION
   static int __read_mostly preempt_thresh = PRI_MAX_IDLE;
   #else
   static int __read_mostly preempt_thresh = PRI_MIN_KERN;
   #endif
   #else 
   static int __read_mostly preempt_thresh = 0;
   #endif
   static int __read_mostly static_boost = PRI_MIN_BATCH;
   static int __read_mostly sched_idlespins = 10000;
   static int __read_mostly sched_idlespinthresh = -1;
   
   /*
    * tdq - per processor runqs and statistics.  A mutex synchronizes access to
    * most fields.  Some fields are loaded or modified without the mutex.
    *
    * Locking protocols:
    * (c)  constant after initialization
    * (f)  flag, set with the tdq lock held, cleared on local CPU
    * (l)  all accesses are CPU-local
    * (ls) stores are performed by the local CPU, loads may be lockless
    * (t)  all accesses are protected by the tdq mutex
    * (ts) stores are serialized by the tdq mutex, loads may be lockless
    */
   struct tdq {
           /* 
            * Ordered to improve efficiency of cpu_search() and switch().
            * tdq_lock is padded to avoid false sharing with tdq_load and
            * tdq_cpu_idle.
            */
           struct mtx_padalign tdq_lock;   /* run queue lock. */
           struct cpu_group *tdq_cg;       /* (c) Pointer to cpu topology. */
           struct thread   *tdq_curthread; /* (t) Current executing thread. */
           int             tdq_load;       /* (ts) Aggregate load. */
           int             tdq_sysload;    /* (ts) For loadavg, !ITHD load. */
           int             tdq_cpu_idle;   /* (ls) cpu_idle() is active. */
           int             tdq_transferable; /* (ts) Transferable thread count. */
           short           tdq_switchcnt;  /* (l) Switches this tick. */
           short           tdq_oldswitchcnt; /* (l) Switches last tick. */
           u_char          tdq_lowpri;     /* (ts) Lowest priority thread. */
           u_char          tdq_owepreempt; /* (f) Remote preemption pending. */
           u_char          tdq_idx;        /* (t) Current insert index. */
           u_char          tdq_ridx;       /* (t) Current removal index. */
           int             tdq_id;         /* (c) cpuid. */
           struct runq     tdq_realtime;   /* (t) real-time run queue. */
           struct runq     tdq_timeshare;  /* (t) timeshare run queue. */
           struct runq     tdq_idle;       /* (t) Queue of IDLE threads. */
           char            tdq_name[TDQ_NAME_LEN];
   #ifdef KTR
           char            tdq_loadname[TDQ_LOADNAME_LEN];
   #endif
   };
   
   /* Idle thread states and config. */
   #define TDQ_RUNNING     1
   #define TDQ_IDLE        2
   
   /* Lockless accessors. */
   #define TDQ_LOAD(tdq)           atomic_load_int(&(tdq)->tdq_load)
   #define TDQ_TRANSFERABLE(tdq)   atomic_load_int(&(tdq)->tdq_transferable)
   #define TDQ_SWITCHCNT(tdq)      (atomic_load_short(&(tdq)->tdq_switchcnt) + \
                                    atomic_load_short(&(tdq)->tdq_oldswitchcnt))
   #define TDQ_SWITCHCNT_INC(tdq)  (atomic_store_short(&(tdq)->tdq_switchcnt, \
                                    atomic_load_short(&(tdq)->tdq_switchcnt) + 1))
   
   #ifdef SMP
   struct cpu_group __read_mostly *cpu_top;                /* CPU topology */
   
   #define SCHED_AFFINITY_DEFAULT  (max(1, hz / 1000))
   #define SCHED_AFFINITY(ts, t)   ((ts)->ts_rltick > ticks - ((t) * affinity))
   
   /*
    * Run-time tunables.
    */
   static int rebalance = 1;
   static int balance_interval = 128;      /* Default set in sched_initticks(). */
   static int __read_mostly affinity;
   static int __read_mostly steal_idle = 1;
   static int __read_mostly steal_thresh = 2;
   static int __read_mostly always_steal = 0;
   static int __read_mostly trysteal_limit = 2;
   
   /*
    * One thread queue per processor.
    */
   static struct tdq __read_mostly *balance_tdq;
   static int balance_ticks;
   DPCPU_DEFINE_STATIC(struct tdq, tdq);
   DPCPU_DEFINE_STATIC(uint32_t, randomval);
   
   #define TDQ_SELF()      ((struct tdq *)PCPU_GET(sched))
   #define TDQ_CPU(x)      (DPCPU_ID_PTR((x), tdq))
   #define TDQ_ID(x)       ((x)->tdq_id)
   #else   /* !SMP */
   static struct tdq       tdq_cpu;
   
   #define TDQ_ID(x)       (0)
   #define TDQ_SELF()      (&tdq_cpu)
   #define TDQ_CPU(x)      (&tdq_cpu)
   #endif
   
   #define TDQ_LOCK_ASSERT(t, type)        mtx_assert(TDQ_LOCKPTR((t)), (type))
   #define TDQ_LOCK(t)             mtx_lock_spin(TDQ_LOCKPTR((t)))
   #define TDQ_LOCK_FLAGS(t, f)    mtx_lock_spin_flags(TDQ_LOCKPTR((t)), (f))
   #define TDQ_TRYLOCK(t)          mtx_trylock_spin(TDQ_LOCKPTR((t)))
   #define TDQ_TRYLOCK_FLAGS(t, f) mtx_trylock_spin_flags(TDQ_LOCKPTR((t)), (f))
   #define TDQ_UNLOCK(t)           mtx_unlock_spin(TDQ_LOCKPTR((t)))
   #define TDQ_LOCKPTR(t)          ((struct mtx *)(&(t)->tdq_lock))
   
   static void sched_setpreempt(int);
   static void sched_priority(struct thread *);
   static void sched_thread_priority(struct thread *, u_char);
   static int sched_interact_score(struct thread *);
   static void sched_interact_update(struct thread *);
   static void sched_interact_fork(struct thread *);
   static void sched_pctcpu_update(struct td_sched *, int);
   
   /* Operations on per processor queues */
   static struct thread *tdq_choose(struct tdq *);
   static void tdq_setup(struct tdq *, int i);
   static void tdq_load_add(struct tdq *, struct thread *);
   static void tdq_load_rem(struct tdq *, struct thread *);
   static __inline void tdq_runq_add(struct tdq *, struct thread *, int);
   static __inline void tdq_runq_rem(struct tdq *, struct thread *);
   static inline int sched_shouldpreempt(int, int, int);
   static void tdq_print(int cpu);
   static void runq_print(struct runq *rq);
   static int tdq_add(struct tdq *, struct thread *, int);
   #ifdef SMP
   static int tdq_move(struct tdq *, struct tdq *);
   static int tdq_idled(struct tdq *);
   static void tdq_notify(struct tdq *, int lowpri);
   static struct thread *tdq_steal(struct tdq *, int);
   static struct thread *runq_steal(struct runq *, int);
   static int sched_pickcpu(struct thread *, int);
   static void sched_balance(void);
   static bool sched_balance_pair(struct tdq *, struct tdq *);
   static inline struct tdq *sched_setcpu(struct thread *, int, int);
   static inline void thread_unblock_switch(struct thread *, struct mtx *);
   static int sysctl_kern_sched_topology_spec(SYSCTL_HANDLER_ARGS);
   static int sysctl_kern_sched_topology_spec_internal(struct sbuf *sb, 
       struct cpu_group *cg, int indent);
   #endif
   
   static void sched_setup(void *dummy);
   SYSINIT(sched_setup, SI_SUB_RUN_QUEUE, SI_ORDER_FIRST, sched_setup, NULL);
   
   static void sched_initticks(void *dummy);
   SYSINIT(sched_initticks, SI_SUB_CLOCKS, SI_ORDER_THIRD, sched_initticks,
       NULL);
   
   SDT_PROVIDER_DEFINE(sched);
   
   SDT_PROBE_DEFINE3(sched, , , change__pri, "struct thread *", 
       "struct proc *", "uint8_t");
   SDT_PROBE_DEFINE3(sched, , , dequeue, "struct thread *", 
       "struct proc *", "void *");
   SDT_PROBE_DEFINE4(sched, , , enqueue, "struct thread *", 
       "struct proc *", "void *", "int");
   SDT_PROBE_DEFINE4(sched, , , lend__pri, "struct thread *", 
       "struct proc *", "uint8_t", "struct thread *");
   SDT_PROBE_DEFINE2(sched, , , load__change, "int", "int");
   SDT_PROBE_DEFINE2(sched, , , off__cpu, "struct thread *", 
       "struct proc *");
   SDT_PROBE_DEFINE(sched, , , on__cpu);
   SDT_PROBE_DEFINE(sched, , , remain__cpu);
   SDT_PROBE_DEFINE2(sched, , , surrender, "struct thread *", 
       "struct proc *");
   
   /*
    * Print the threads waiting on a run-queue.
    */
   static void
   runq_print(struct runq *rq)
   {
           struct rqhead *rqh;
           struct thread *td;
           int pri;
           int j;
           int i;
   
           for (i = 0; i < RQB_LEN; i++) {
                   printf("\t\trunq bits %d 0x%zx\n",
                       i, rq->rq_status.rqb_bits[i]);
                   for (j = 0; j < RQB_BPW; j++)
                           if (rq->rq_status.rqb_bits[i] & (1ul << j)) {
                                   pri = j + (i << RQB_L2BPW);
                                   rqh = &rq->rq_queues[pri];
                                   TAILQ_FOREACH(td, rqh, td_runq) {
                                           printf("\t\t\ttd %p(%s) priority %d rqindex %d pri %d\n",
                                               td, td->td_name, td->td_priority,
                                               td->td_rqindex, pri);
                                   }
                           }
           }
   }
   
   /*
    * Print the status of a per-cpu thread queue.  Should be a ddb show cmd.
    */
   static void __unused
   tdq_print(int cpu)
   {
           struct tdq *tdq;
   
           tdq = TDQ_CPU(cpu);
   
           printf("tdq %d:\n", TDQ_ID(tdq));
           printf("\tlock            %p\n", TDQ_LOCKPTR(tdq));
           printf("\tLock name:      %s\n", tdq->tdq_name);
           printf("\tload:           %d\n", tdq->tdq_load);
           printf("\tswitch cnt:     %d\n", tdq->tdq_switchcnt);
           printf("\told switch cnt: %d\n", tdq->tdq_oldswitchcnt);
           printf("\ttimeshare idx:  %d\n", tdq->tdq_idx);
           printf("\ttimeshare ridx: %d\n", tdq->tdq_ridx);
           printf("\tload transferable: %d\n", tdq->tdq_transferable);
           printf("\tlowest priority:   %d\n", tdq->tdq_lowpri);
           printf("\trealtime runq:\n");
           runq_print(&tdq->tdq_realtime);
           printf("\ttimeshare runq:\n");
           runq_print(&tdq->tdq_timeshare);
           printf("\tidle runq:\n");
           runq_print(&tdq->tdq_idle);
   }
   
   static inline int
   sched_shouldpreempt(int pri, int cpri, int remote)
   {
           /*
            * If the new priority is not better than the current priority there is
            * nothing to do.
            */
           if (pri >= cpri)
                   return (0);
           /*
            * Always preempt idle.
            */
           if (cpri >= PRI_MIN_IDLE)
                   return (1);
           /*
            * If preemption is disabled don't preempt others.
            */
           if (preempt_thresh == 0)
                   return (0);
           /*
            * Preempt if we exceed the threshold.
            */
           if (pri <= preempt_thresh)
                   return (1);
           /*
            * If we're interactive or better and there is non-interactive
            * or worse running preempt only remote processors.
            */
           if (remote && pri <= PRI_MAX_INTERACT && cpri > PRI_MAX_INTERACT)
                   return (1);
           return (0);
   }
   
   /*
    * Add a thread to the actual run-queue.  Keeps transferable counts up to
    * date with what is actually on the run-queue.  Selects the correct
    * queue position for timeshare threads.
    */
   static __inline void
   tdq_runq_add(struct tdq *tdq, struct thread *td, int flags)
   {
           struct td_sched *ts;
           u_char pri;
   
           TDQ_LOCK_ASSERT(tdq, MA_OWNED);
           THREAD_LOCK_BLOCKED_ASSERT(td, MA_OWNED);
   
           pri = td->td_priority;
           ts = td_get_sched(td);
           TD_SET_RUNQ(td);
           if (THREAD_CAN_MIGRATE(td)) {
                   tdq->tdq_transferable++;
                   ts->ts_flags |= TSF_XFERABLE;
           }
           if (pri < PRI_MIN_BATCH) {
                   ts->ts_runq = &tdq->tdq_realtime;
           } else if (pri <= PRI_MAX_BATCH) {
                   ts->ts_runq = &tdq->tdq_timeshare;
                   KASSERT(pri <= PRI_MAX_BATCH && pri >= PRI_MIN_BATCH,
                           ("Invalid priority %d on timeshare runq", pri));
                   /*
                    * This queue contains only priorities between MIN and MAX
                    * batch.  Use the whole queue to represent these values.
                    */
                   if ((flags & (SRQ_BORROWING|SRQ_PREEMPTED)) == 0) {
                           pri = RQ_NQS * (pri - PRI_MIN_BATCH) / PRI_BATCH_RANGE;
                           pri = (pri + tdq->tdq_idx) % RQ_NQS;
                           /*
                            * This effectively shortens the queue by one so we
                            * can have a one slot difference between idx and
                            * ridx while we wait for threads to drain.
                            */
                           if (tdq->tdq_ridx != tdq->tdq_idx &&
                               pri == tdq->tdq_ridx)
                                   pri = (unsigned char)(pri - 1) % RQ_NQS;
                   } else
                           pri = tdq->tdq_ridx;
                   runq_add_pri(ts->ts_runq, td, pri, flags);
                   return;
           } else
                   ts->ts_runq = &tdq->tdq_idle;
           runq_add(ts->ts_runq, td, flags);
   }
   
   /* 
    * Remove a thread from a run-queue.  This typically happens when a thread
    * is selected to run.  Running threads are not on the queue and the
    * transferable count does not reflect them.
    */
   static __inline void
   tdq_runq_rem(struct tdq *tdq, struct thread *td)
   {
           struct td_sched *ts;
   
           ts = td_get_sched(td);
           TDQ_LOCK_ASSERT(tdq, MA_OWNED);
           THREAD_LOCK_BLOCKED_ASSERT(td, MA_OWNED);
           KASSERT(ts->ts_runq != NULL,
               ("tdq_runq_remove: thread %p null ts_runq", td));
           if (ts->ts_flags & TSF_XFERABLE) {
                   tdq->tdq_transferable--;
                   ts->ts_flags &= ~TSF_XFERABLE;
           }
           if (ts->ts_runq == &tdq->tdq_timeshare) {
                   if (tdq->tdq_idx != tdq->tdq_ridx)
                           runq_remove_idx(ts->ts_runq, td, &tdq->tdq_ridx);
                   else
                           runq_remove_idx(ts->ts_runq, td, NULL);
           } else
                   runq_remove(ts->ts_runq, td);
   }
   
   /*
    * Load is maintained for all threads RUNNING and ON_RUNQ.  Add the load
    * for this thread to the referenced thread queue.
    */
   static void
   tdq_load_add(struct tdq *tdq, struct thread *td)
   {
   
           TDQ_LOCK_ASSERT(tdq, MA_OWNED);
           THREAD_LOCK_BLOCKED_ASSERT(td, MA_OWNED);
   
           tdq->tdq_load++;
           if ((td->td_flags & TDF_NOLOAD) == 0)
                   tdq->tdq_sysload++;
           KTR_COUNTER0(KTR_SCHED, "load", tdq->tdq_loadname, tdq->tdq_load);
           SDT_PROBE2(sched, , , load__change, (int)TDQ_ID(tdq), tdq->tdq_load);
   }
   
   /*
    * Remove the load from a thread that is transitioning to a sleep state or
    * exiting.
    */
   static void
   tdq_load_rem(struct tdq *tdq, struct thread *td)
   {
   
           TDQ_LOCK_ASSERT(tdq, MA_OWNED);
           THREAD_LOCK_BLOCKED_ASSERT(td, MA_OWNED);
           KASSERT(tdq->tdq_load != 0,
               ("tdq_load_rem: Removing with 0 load on queue %d", TDQ_ID(tdq)));
   
           tdq->tdq_load--;
           if ((td->td_flags & TDF_NOLOAD) == 0)
                   tdq->tdq_sysload--;
           KTR_COUNTER0(KTR_SCHED, "load", tdq->tdq_loadname, tdq->tdq_load);
           SDT_PROBE2(sched, , , load__change, (int)TDQ_ID(tdq), tdq->tdq_load);
   }
   
   /*
    * Bound timeshare latency by decreasing slice size as load increases.  We
    * consider the maximum latency as the sum of the threads waiting to run
    * aside from curthread and target no more than sched_slice latency but
    * no less than sched_slice_min runtime.
    */
   static inline int
   tdq_slice(struct tdq *tdq)
   {
           int load;
   
           /*
            * It is safe to use sys_load here because this is called from
            * contexts where timeshare threads are running and so there
            * cannot be higher priority load in the system.
            */
           load = tdq->tdq_sysload - 1;
           if (load >= SCHED_SLICE_MIN_DIVISOR)
                   return (sched_slice_min);
           if (load <= 1)
                   return (sched_slice);
           return (sched_slice / load);
   }
   
   /*
    * Set lowpri to its exact value by searching the run-queue and
    * evaluating curthread.  curthread may be passed as an optimization.
    */
   static void
   tdq_setlowpri(struct tdq *tdq, struct thread *ctd)
   {
           struct thread *td;
   
           TDQ_LOCK_ASSERT(tdq, MA_OWNED);
           if (ctd == NULL)
                   ctd = tdq->tdq_curthread;
           td = tdq_choose(tdq);
           if (td == NULL || td->td_priority > ctd->td_priority)
                   tdq->tdq_lowpri = ctd->td_priority;
           else
                   tdq->tdq_lowpri = td->td_priority;
   }
   
   #ifdef SMP
   /*
    * We need some randomness. Implement a classic Linear Congruential
    * Generator X_{n+1}=(aX_n+c) mod m. These values are optimized for
    * m = 2^32, a = 69069 and c = 5. We only return the upper 16 bits
    * of the random state (in the low bits of our answer) to keep
    * the maximum randomness.
    */
   static uint32_t
   sched_random(void)
   {
           uint32_t *rndptr;
   
           rndptr = DPCPU_PTR(randomval);
           *rndptr = *rndptr * 69069 + 5;
   
           return (*rndptr >> 16);
   }
   
   struct cpu_search {
           cpuset_t *cs_mask;      /* The mask of allowed CPUs to choose from. */
           int     cs_prefer;      /* Prefer this CPU and groups including it. */
           int     cs_running;     /* The thread is now running at cs_prefer. */
           int     cs_pri;         /* Min priority for low. */
           int     cs_load;        /* Max load for low, min load for high. */
           int     cs_trans;       /* Min transferable load for high. */
   };
   
   struct cpu_search_res {
           int     csr_cpu;        /* The best CPU found. */
           int     csr_load;       /* The load of cs_cpu. */
   };
   
   /*
    * Search the tree of cpu_groups for the lowest or highest loaded CPU.
    * These routines actually compare the load on all paths through the tree
    * and find the least loaded cpu on the least loaded path, which may differ
    * from the least loaded cpu in the system.  This balances work among caches
    * and buses.
    */
   static int
   cpu_search_lowest(const struct cpu_group *cg, const struct cpu_search *s,
       struct cpu_search_res *r)
   {
           struct cpu_search_res lr;
           struct tdq *tdq;
           int c, bload, l, load, p, total;
   
           total = 0;
           bload = INT_MAX;
           r->csr_cpu = -1;
   
           /* Loop through children CPU groups if there are any. */
           if (cg->cg_children > 0) {
                   for (c = cg->cg_children - 1; c >= 0; c--) {
                           load = cpu_search_lowest(&cg->cg_child[c], s, &lr);
                           total += load;
   
                           /*
                            * When balancing do not prefer SMT groups with load >1.
                            * It allows round-robin between SMT groups with equal
                            * load within parent group for more fair scheduling.
                            */
                           if (__predict_false(s->cs_running) &&
                               (cg->cg_child[c].cg_flags & CG_FLAG_THREAD) &&
                               load >= 128 && (load & 128) != 0)
                                   load += 128;
   
                           if (lr.csr_cpu >= 0 && (load < bload ||
                               (load == bload && lr.csr_load < r->csr_load))) {
                                   bload = load;
                                   r->csr_cpu = lr.csr_cpu;
                                   r->csr_load = lr.csr_load;
                           }
                   }
                   return (total);
           }
   
           /* Loop through children CPUs otherwise. */
           for (c = cg->cg_last; c >= cg->cg_first; c--) {
                   if (!CPU_ISSET(c, &cg->cg_mask))
                           continue;
                   tdq = TDQ_CPU(c);
                   l = TDQ_LOAD(tdq);
                   if (c == s->cs_prefer) {
                           if (__predict_false(s->cs_running))
                                   l--;
                           p = 128;
                   } else
                           p = 0;
                   load = l * 256;
                   total += load - p;
   
                   /*
                    * Check this CPU is acceptable.
                    * If the threads is already on the CPU, don't look on the TDQ
                    * priority, since it can be the priority of the thread itself.
                    */
                   if (l > s->cs_load ||
                       (atomic_load_char(&tdq->tdq_lowpri) <= s->cs_pri &&
                        (!s->cs_running || c != s->cs_prefer)) ||
                       !CPU_ISSET(c, s->cs_mask))
                           continue;
   
                   /*
                    * When balancing do not prefer CPUs with load > 1.
                    * It allows round-robin between CPUs with equal load
                    * within the CPU group for more fair scheduling.
                    */
                   if (__predict_false(s->cs_running) && l > 0)
                           p = 0;
   
                   load -= sched_random() % 128;
                   if (bload > load - p) {
                           bload = load - p;
                           r->csr_cpu = c;
                           r->csr_load = load;
                   }
           }
           return (total);
   }
   
   static int
   cpu_search_highest(const struct cpu_group *cg, const struct cpu_search *s,
       struct cpu_search_res *r)
   {
           struct cpu_search_res lr;
           struct tdq *tdq;
           int c, bload, l, load, total;
   
           total = 0;
           bload = INT_MIN;
           r->csr_cpu = -1;
   
           /* Loop through children CPU groups if there are any. */
           if (cg->cg_children > 0) {
                   for (c = cg->cg_children - 1; c >= 0; c--) {
                           load = cpu_search_highest(&cg->cg_child[c], s, &lr);
                           total += load;
                           if (lr.csr_cpu >= 0 && (load > bload ||
                               (load == bload && lr.csr_load > r->csr_load))) {
                                   bload = load;
                                   r->csr_cpu = lr.csr_cpu;
                                   r->csr_load = lr.csr_load;
                           }
                   }
                   return (total);
           }
   
           /* Loop through children CPUs otherwise. */
           for (c = cg->cg_last; c >= cg->cg_first; c--) {
                   if (!CPU_ISSET(c, &cg->cg_mask))
                           continue;
                   tdq = TDQ_CPU(c);
                   l = TDQ_LOAD(tdq);
                   load = l * 256;
                   total += load;
   
                   /*
                    * Check this CPU is acceptable.
                    */
                   if (l < s->cs_load || TDQ_TRANSFERABLE(tdq) < s->cs_trans ||
                       !CPU_ISSET(c, s->cs_mask))
                           continue;
   
                   load -= sched_random() % 256;
                   if (load > bload) {
                           bload = load;
                           r->csr_cpu = c;
                   }
           }
           r->csr_load = bload;
           return (total);
   }
   
   /*
    * Find the cpu with the least load via the least loaded path that has a
    * lowpri greater than pri  pri.  A pri of -1 indicates any priority is
    * acceptable.
    */
   static inline int
   sched_lowest(const struct cpu_group *cg, cpuset_t *mask, int pri, int maxload,
       int prefer, int running)
   {
           struct cpu_search s;
           struct cpu_search_res r;
   
           s.cs_prefer = prefer;
           s.cs_running = running;
           s.cs_mask = mask;
           s.cs_pri = pri;
           s.cs_load = maxload;
           cpu_search_lowest(cg, &s, &r);
           return (r.csr_cpu);
   }
   
   /*
    * Find the cpu with the highest load via the highest loaded path.
    */
   static inline int
   sched_highest(const struct cpu_group *cg, cpuset_t *mask, int minload,
       int mintrans)
   {
           struct cpu_search s;
           struct cpu_search_res r;
   
           s.cs_mask = mask;
           s.cs_load = minload;
           s.cs_trans = mintrans;
           cpu_search_highest(cg, &s, &r);
           return (r.csr_cpu);
   }
   
   static void
   sched_balance_group(struct cpu_group *cg)
   {
           struct tdq *tdq;
           struct thread *td;
           cpuset_t hmask, lmask;
           int high, low, anylow;
   
           CPU_FILL(&hmask);
           for (;;) {
                   high = sched_highest(cg, &hmask, 1, 0);
                   /* Stop if there is no more CPU with transferrable threads. */
                   if (high == -1)
                           break;
                   CPU_CLR(high, &hmask);
                   CPU_COPY(&hmask, &lmask);
                   /* Stop if there is no more CPU left for low. */
                   if (CPU_EMPTY(&lmask))
                           break;
                   tdq = TDQ_CPU(high);
                   if (TDQ_LOAD(tdq) == 1) {
                           /*
                            * There is only one running thread.  We can't move
                            * it from here, so tell it to pick new CPU by itself.
                            */
                           TDQ_LOCK(tdq);
                           td = tdq->tdq_curthread;
                           if (td->td_lock == TDQ_LOCKPTR(tdq) &&
                               (td->td_flags & TDF_IDLETD) == 0 &&
                               THREAD_CAN_MIGRATE(td)) {
                                   td->td_flags |= TDF_PICKCPU;
                                   ast_sched_locked(td, TDA_SCHED);
                                   if (high != curcpu)
                                           ipi_cpu(high, IPI_AST);
                           }
                           TDQ_UNLOCK(tdq);
                           break;
                   }
                   anylow = 1;
   nextlow:
                   if (TDQ_TRANSFERABLE(tdq) == 0)
                           continue;
                   low = sched_lowest(cg, &lmask, -1, TDQ_LOAD(tdq) - 1, high, 1);
                   /* Stop if we looked well and found no less loaded CPU. */
                   if (anylow && low == -1)
                           break;
                   /* Go to next high if we found no less loaded CPU. */
                   if (low == -1)
                           continue;
                   /* Transfer thread from high to low. */
                   if (sched_balance_pair(tdq, TDQ_CPU(low))) {
                           /* CPU that got thread can no longer be a donor. */
                           CPU_CLR(low, &hmask);
                   } else {
                           /*
                            * If failed, then there is no threads on high
                            * that can run on this low. Drop low from low
                            * mask and look for different one.
                            */
                           CPU_CLR(low, &lmask);
                           anylow = 0;
                           goto nextlow;
                   }
           }
   }
   
   static void
   sched_balance(void)
   {
           struct tdq *tdq;
   
           balance_ticks = max(balance_interval / 2, 1) +
               (sched_random() % balance_interval);
           tdq = TDQ_SELF();
           TDQ_UNLOCK(tdq);
           sched_balance_group(cpu_top);
           TDQ_LOCK(tdq);
   }
   
   /*
    * Lock two thread queues using their address to maintain lock order.
    */
   static void
   tdq_lock_pair(struct tdq *one, struct tdq *two)
   {
           if (one < two) {
                   TDQ_LOCK(one);
                   TDQ_LOCK_FLAGS(two, MTX_DUPOK);
           } else {
                   TDQ_LOCK(two);
                   TDQ_LOCK_FLAGS(one, MTX_DUPOK);
           }
   }
   
   /*
    * Unlock two thread queues.  Order is not important here.
    */
   static void
   tdq_unlock_pair(struct tdq *one, struct tdq *two)
   {
           TDQ_UNLOCK(one);
           TDQ_UNLOCK(two);
   }
   
   /*
    * Transfer load between two imbalanced thread queues.  Returns true if a thread
    * was moved between the queues, and false otherwise.
    */
   static bool
   sched_balance_pair(struct tdq *high, struct tdq *low)
   {
           int cpu, lowpri;
           bool ret;
   
           ret = false;
           tdq_lock_pair(high, low);
   
           /*
            * Transfer a thread from high to low.
            */
           if (high->tdq_transferable != 0 && high->tdq_load > low->tdq_load) {
                   lowpri = tdq_move(high, low);
                   if (lowpri != -1) {
                           /*
                            * In case the target isn't the current CPU notify it of
                            * the new load, possibly sending an IPI to force it to
                            * reschedule.  Otherwise maybe schedule a preemption.
                            */
                           cpu = TDQ_ID(low);
                           if (cpu != PCPU_GET(cpuid))
                                   tdq_notify(low, lowpri);
                           else
                                   sched_setpreempt(low->tdq_lowpri);
                           ret = true;
                   }
           }
           tdq_unlock_pair(high, low);
           return (ret);
   }
   
   /*
    * Move a thread from one thread queue to another.  Returns -1 if the source
    * queue was empty, else returns the maximum priority of all threads in
    * the destination queue prior to the addition of the new thread.  In the latter
    * case, this priority can be used to determine whether an IPI needs to be
    * delivered.
    */
   static int
   tdq_move(struct tdq *from, struct tdq *to)
   {
           struct thread *td;
           int cpu;
   
          TDQ_LOCK_ASSERT(from, MA_OWNED);
          TDQ_LOCK_ASSERT(to, MA_OWNED);
  
          cpu = TDQ_ID(to);
          td = tdq_steal(from, cpu);
          if (td == NULL)
                  return (-1);
  
          /*
           * Although the run queue is locked the thread may be
           * blocked.  We can not set the lock until it is unblocked.
           */
          thread_lock_block_wait(td);
          sched_rem(td);
          THREAD_LOCKPTR_ASSERT(td, TDQ_LOCKPTR(from));
          td->td_lock = TDQ_LOCKPTR(to);
          td_get_sched(td)->ts_cpu = cpu;
          return (tdq_add(to, td, SRQ_YIELDING));
  }
  
  /*
   * This tdq has idled.  Try to steal a thread from another cpu and switch
   * to it.
   */
  static int
  tdq_idled(struct tdq *tdq)
  {
          struct cpu_group *cg, *parent;
          struct tdq *steal;
          cpuset_t mask;
          int cpu, switchcnt, goup;
  
          if (smp_started == 0 || steal_idle == 0 || tdq->tdq_cg == NULL)
                  return (1);
          CPU_FILL(&mask);
          CPU_CLR(PCPU_GET(cpuid), &mask);
  restart:
          switchcnt = TDQ_SWITCHCNT(tdq);
          for (cg = tdq->tdq_cg, goup = 0; ; ) {
                  cpu = sched_highest(cg, &mask, steal_thresh, 1);
                  /*
                   * We were assigned a thread but not preempted.  Returning
                   * 0 here will cause our caller to switch to it.
                   */
                  if (TDQ_LOAD(tdq))
                          return (0);
  
                  /*
                   * We found no CPU to steal from in this group.  Escalate to
                   * the parent and repeat.  But if parent has only two children
                   * groups we can avoid searching this group again by searching
                   * the other one specifically and then escalating two levels.
                   */
                  if (cpu == -1) {
                          if (goup) {
                                  cg = cg->cg_parent;
                                  goup = 0;
                          }
                          parent = cg->cg_parent;
                          if (parent == NULL)
                                  return (1);
                          if (parent->cg_children == 2) {
                                  if (cg == &parent->cg_child[0])
                                          cg = &parent->cg_child[1];
                                  else
                                          cg = &parent->cg_child[0];
                                  goup = 1;
                          } else
                                  cg = parent;
                          continue;
                  }
                  steal = TDQ_CPU(cpu);
                  /*
                   * The data returned by sched_highest() is stale and
                   * the chosen CPU no longer has an eligible thread.
                   *
                   * Testing this ahead of tdq_lock_pair() only catches
                   * this situation about 20% of the time on an 8 core
                   * 16 thread Ryzen 7, but it still helps performance.
                   */
                  if (TDQ_LOAD(steal) < steal_thresh ||
                      TDQ_TRANSFERABLE(steal) == 0)
                          goto restart;
                  /*
                   * Try to lock both queues. If we are assigned a thread while
                   * waited for the lock, switch to it now instead of stealing.
                   * If we can't get the lock, then somebody likely got there
                   * first so continue searching.
                   */
                  TDQ_LOCK(tdq);
                  if (tdq->tdq_load > 0) {
                          mi_switch(SW_VOL | SWT_IDLE);
                          return (0);
                  }
                  if (TDQ_TRYLOCK_FLAGS(steal, MTX_DUPOK) == 0) {
                          TDQ_UNLOCK(tdq);
                          CPU_CLR(cpu, &mask);
                          continue;
                  }
                  /*
                   * The data returned by sched_highest() is stale and
                   * the chosen CPU no longer has an eligible thread, or
                   * we were preempted and the CPU loading info may be out
                   * of date.  The latter is rare.  In either case restart
                   * the search.
                   */
                  if (TDQ_LOAD(steal) < steal_thresh ||
                      TDQ_TRANSFERABLE(steal) == 0 ||
                      switchcnt != TDQ_SWITCHCNT(tdq)) {
                          tdq_unlock_pair(tdq, steal);
                          goto restart;
                  }
                  /*
                   * Steal the thread and switch to it.
                   */
                  if (tdq_move(steal, tdq) != -1)
                          break;
                  /*
                   * We failed to acquire a thread even though it looked
                   * like one was available.  This could be due to affinity
                   * restrictions or for other reasons.  Loop again after
                   * removing this CPU from the set.  The restart logic
                   * above does not restore this CPU to the set due to the
                   * likelyhood of failing here again.
                   */
                  CPU_CLR(cpu, &mask);
                  tdq_unlock_pair(tdq, steal);
          }
          TDQ_UNLOCK(steal);
          mi_switch(SW_VOL | SWT_IDLE);
          return (0);
  }
  
  /*
   * Notify a remote cpu of new work.  Sends an IPI if criteria are met.
   *
   * "lowpri" is the minimum scheduling priority among all threads on
   * the queue prior to the addition of the new thread.
   */
  static void
  tdq_notify(struct tdq *tdq, int lowpri)
  {
          int cpu;
  
          TDQ_LOCK_ASSERT(tdq, MA_OWNED);
          KASSERT(tdq->tdq_lowpri <= lowpri,
              ("tdq_notify: lowpri %d > tdq_lowpri %d", lowpri, tdq->tdq_lowpri));
  
          if (tdq->tdq_owepreempt)
                  return;
  
          /*
           * Check to see if the newly added thread should preempt the one
           * currently running.
           */
          if (!sched_shouldpreempt(tdq->tdq_lowpri, lowpri, 1))
                  return;
  
          /*
           * Make sure that our caller's earlier update to tdq_load is
           * globally visible before we read tdq_cpu_idle.  Idle thread
           * accesses both of them without locks, and the order is important.
           */
          atomic_thread_fence_seq_cst();
  
          /*
           * Try to figure out if we can signal the idle thread instead of sending
           * an IPI.  This check is racy; at worst, we will deliever an IPI
           * unnecessarily.
           */
          cpu = TDQ_ID(tdq);
          if (TD_IS_IDLETHREAD(tdq->tdq_curthread) &&
              (atomic_load_int(&tdq->tdq_cpu_idle) == 0 || cpu_idle_wakeup(cpu)))
                  return;
  
          /*
           * The run queues have been updated, so any switch on the remote CPU
           * will satisfy the preemption request.
           */
          tdq->tdq_owepreempt = 1;
          ipi_cpu(cpu, IPI_PREEMPT);
  }
  
  /*
   * Steals load from a timeshare queue.  Honors the rotating queue head
   * index.
   */
  static struct thread *
  runq_steal_from(struct runq *rq, int cpu, u_char start)
  {
          struct rqbits *rqb;
          struct rqhead *rqh;
          struct thread *td, *first;
          int bit;
          int i;
  
          rqb = &rq->rq_status;
          bit = start & (RQB_BPW -1);
          first = NULL;
  again:
          for (i = RQB_WORD(start); i < RQB_LEN; bit = 0, i++) {
                  if (rqb->rqb_bits[i] == 0)
                          continue;
                  if (bit == 0)
                          bit = RQB_FFS(rqb->rqb_bits[i]);
                  for (; bit < RQB_BPW; bit++) {
                          if ((rqb->rqb_bits[i] & (1ul << bit)) == 0)
                                  continue;
                          rqh = &rq->rq_queues[bit + (i << RQB_L2BPW)];
                          TAILQ_FOREACH(td, rqh, td_runq) {
                                  if (first) {
                                          if (THREAD_CAN_MIGRATE(td) &&
                                              THREAD_CAN_SCHED(td, cpu))
                                                  return (td);
                                  } else
                                          first = td;
                          }
                  }
          }
          if (start != 0) {
                  start = 0;
                  goto again;
          }
  
          if (first && THREAD_CAN_MIGRATE(first) &&
              THREAD_CAN_SCHED(first, cpu))
                  return (first);
          return (NULL);
  }
  
  /*
   * Steals load from a standard linear queue.
   */
  static struct thread *
  runq_steal(struct runq *rq, int cpu)
  {
          struct rqhead *rqh;
          struct rqbits *rqb;
          struct thread *td;
          int word;
          int bit;
  
          rqb = &rq->rq_status;
          for (word = 0; word < RQB_LEN; word++) {
                  if (rqb->rqb_bits[word] == 0)
                          continue;
                  for (bit = 0; bit < RQB_BPW; bit++) {
                          if ((rqb->rqb_bits[word] & (1ul << bit)) == 0)
                                  continue;
                          rqh = &rq->rq_queues[bit + (word << RQB_L2BPW)];
                          TAILQ_FOREACH(td, rqh, td_runq)
                                  if (THREAD_CAN_MIGRATE(td) &&
                                      THREAD_CAN_SCHED(td, cpu))
                                          return (td);
                  }
          }
          return (NULL);
  }
  
  /*
   * Attempt to steal a thread in priority order from a thread queue.
   */
  static struct thread *
  tdq_steal(struct tdq *tdq, int cpu)
  {
          struct thread *td;
  
          TDQ_LOCK_ASSERT(tdq, MA_OWNED);
          if ((td = runq_steal(&tdq->tdq_realtime, cpu)) != NULL)
                  return (td);
          if ((td = runq_steal_from(&tdq->tdq_timeshare,
              cpu, tdq->tdq_ridx)) != NULL)
                  return (td);
          return (runq_steal(&tdq->tdq_idle, cpu));
  }
  
  /*
   * Sets the thread lock and ts_cpu to match the requested cpu.  Unlocks the
   * current lock and returns with the assigned queue locked.
   */
  static inline struct tdq *
  sched_setcpu(struct thread *td, int cpu, int flags)
  {
  
          struct tdq *tdq;
          struct mtx *mtx;
  
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          tdq = TDQ_CPU(cpu);
          td_get_sched(td)->ts_cpu = cpu;
          /*
           * If the lock matches just return the queue.
           */
          if (td->td_lock == TDQ_LOCKPTR(tdq)) {
                  KASSERT((flags & SRQ_HOLD) == 0,
                      ("sched_setcpu: Invalid lock for SRQ_HOLD"));
                  return (tdq);
          }
  
          /*
           * The hard case, migration, we need to block the thread first to
           * prevent order reversals with other cpus locks.
           */
          spinlock_enter();
          mtx = thread_lock_block(td);
          if ((flags & SRQ_HOLD) == 0)
                  mtx_unlock_spin(mtx);
          TDQ_LOCK(tdq);
          thread_lock_unblock(td, TDQ_LOCKPTR(tdq));
          spinlock_exit();
          return (tdq);
  }
  
  SCHED_STAT_DEFINE(pickcpu_intrbind, "Soft interrupt binding");
  SCHED_STAT_DEFINE(pickcpu_idle_affinity, "Picked idle cpu based on affinity");
  SCHED_STAT_DEFINE(pickcpu_affinity, "Picked cpu based on affinity");
  SCHED_STAT_DEFINE(pickcpu_lowest, "Selected lowest load");
  SCHED_STAT_DEFINE(pickcpu_local, "Migrated to current cpu");
  SCHED_STAT_DEFINE(pickcpu_migration, "Selection may have caused migration");
  
  static int
  sched_pickcpu(struct thread *td, int flags)
  {
          struct cpu_group *cg, *ccg;
          struct td_sched *ts;
          struct tdq *tdq;
          cpuset_t *mask;
          int cpu, pri, r, self, intr;
  
          self = PCPU_GET(cpuid);
          ts = td_get_sched(td);
          KASSERT(!CPU_ABSENT(ts->ts_cpu), ("sched_pickcpu: Start scheduler on "
              "absent CPU %d for thread %s.", ts->ts_cpu, td->td_name));
          if (smp_started == 0)
                  return (self);
          /*
           * Don't migrate a running thread from sched_switch().
           */
          if ((flags & SRQ_OURSELF) || !THREAD_CAN_MIGRATE(td))
                  return (ts->ts_cpu);
          /*
           * Prefer to run interrupt threads on the processors that generate
           * the interrupt.
           */
          if (td->td_priority <= PRI_MAX_ITHD && THREAD_CAN_SCHED(td, self) &&
              curthread->td_intr_nesting_level) {
                  tdq = TDQ_SELF();
                  if (tdq->tdq_lowpri >= PRI_MIN_IDLE) {
                          SCHED_STAT_INC(pickcpu_idle_affinity);
                          return (self);
                  }
                  ts->ts_cpu = self;
                  intr = 1;
                  cg = tdq->tdq_cg;
                  goto llc;
          } else {
                  intr = 0;
                  tdq = TDQ_CPU(ts->ts_cpu);
                  cg = tdq->tdq_cg;
          }
          /*
           * If the thread can run on the last cpu and the affinity has not
           * expired and it is idle, run it there.
           */
          if (THREAD_CAN_SCHED(td, ts->ts_cpu) &&
              atomic_load_char(&tdq->tdq_lowpri) >= PRI_MIN_IDLE &&
              SCHED_AFFINITY(ts, CG_SHARE_L2)) {
                  if (cg->cg_flags & CG_FLAG_THREAD) {
                          /* Check all SMT threads for being idle. */
                          for (cpu = cg->cg_first; cpu <= cg->cg_last; cpu++) {
                                  pri =
                                      atomic_load_char(&TDQ_CPU(cpu)->tdq_lowpri);
                                  if (CPU_ISSET(cpu, &cg->cg_mask) &&
                                      pri < PRI_MIN_IDLE)
                                          break;
                          }
                          if (cpu > cg->cg_last) {
                                  SCHED_STAT_INC(pickcpu_idle_affinity);
                                  return (ts->ts_cpu);
                          }
                  } else {
                          SCHED_STAT_INC(pickcpu_idle_affinity);
                          return (ts->ts_cpu);
                  }
          }
  llc:
          /*
           * Search for the last level cache CPU group in the tree.
           * Skip SMT, identical groups and caches with expired affinity.
           * Interrupt threads affinity is explicit and never expires.
           */
          for (ccg = NULL; cg != NULL; cg = cg->cg_parent) {
                  if (cg->cg_flags & CG_FLAG_THREAD)
                          continue;
                  if (cg->cg_children == 1 || cg->cg_count == 1)
                          continue;
                  if (cg->cg_level == CG_SHARE_NONE ||
                      (!intr && !SCHED_AFFINITY(ts, cg->cg_level)))
                          continue;
                  ccg = cg;
          }
          /* Found LLC shared by all CPUs, so do a global search. */
          if (ccg == cpu_top)
                  ccg = NULL;
          cpu = -1;
          mask = &td->td_cpuset->cs_mask;
          pri = td->td_priority;
          r = TD_IS_RUNNING(td);
          /*
           * Try hard to keep interrupts within found LLC.  Search the LLC for
           * the least loaded CPU we can run now.  For NUMA systems it should
           * be within target domain, and it also reduces scheduling overhead.
           */
          if (ccg != NULL && intr) {
                  cpu = sched_lowest(ccg, mask, pri, INT_MAX, ts->ts_cpu, r);
                  if (cpu >= 0)
                          SCHED_STAT_INC(pickcpu_intrbind);
          } else
          /* Search the LLC for the least loaded idle CPU we can run now. */
          if (ccg != NULL) {
                  cpu = sched_lowest(ccg, mask, max(pri, PRI_MAX_TIMESHARE),
                      INT_MAX, ts->ts_cpu, r);
                  if (cpu >= 0)
                          SCHED_STAT_INC(pickcpu_affinity);
          }
          /* Search globally for the least loaded CPU we can run now. */
          if (cpu < 0) {
                  cpu = sched_lowest(cpu_top, mask, pri, INT_MAX, ts->ts_cpu, r);
                  if (cpu >= 0)
                          SCHED_STAT_INC(pickcpu_lowest);
          }
          /* Search globally for the least loaded CPU. */
          if (cpu < 0) {
                  cpu = sched_lowest(cpu_top, mask, -1, INT_MAX, ts->ts_cpu, r);
                  if (cpu >= 0)
                          SCHED_STAT_INC(pickcpu_lowest);
          }
          KASSERT(cpu >= 0, ("sched_pickcpu: Failed to find a cpu."));
          KASSERT(!CPU_ABSENT(cpu), ("sched_pickcpu: Picked absent CPU %d.", cpu));
          /*
           * Compare the lowest loaded cpu to current cpu.
           */
          tdq = TDQ_CPU(cpu);
          if (THREAD_CAN_SCHED(td, self) && TDQ_SELF()->tdq_lowpri > pri &&
              atomic_load_char(&tdq->tdq_lowpri) < PRI_MIN_IDLE &&
              TDQ_LOAD(TDQ_SELF()) <= TDQ_LOAD(tdq) + 1) {
                  SCHED_STAT_INC(pickcpu_local);
                  cpu = self;
          }
          if (cpu != ts->ts_cpu)
                  SCHED_STAT_INC(pickcpu_migration);
          return (cpu);
  }
  #endif
  
  /*
   * Pick the highest priority task we have and return it.
   */
  static struct thread *
  tdq_choose(struct tdq *tdq)
  {
          struct thread *td;
  
          TDQ_LOCK_ASSERT(tdq, MA_OWNED);
          td = runq_choose(&tdq->tdq_realtime);
          if (td != NULL)
                  return (td);
          td = runq_choose_from(&tdq->tdq_timeshare, tdq->tdq_ridx);
          if (td != NULL) {
                  KASSERT(td->td_priority >= PRI_MIN_BATCH,
                      ("tdq_choose: Invalid priority on timeshare queue %d",
                      td->td_priority));
                  return (td);
          }
          td = runq_choose(&tdq->tdq_idle);
          if (td != NULL) {
                  KASSERT(td->td_priority >= PRI_MIN_IDLE,
                      ("tdq_choose: Invalid priority on idle queue %d",
                      td->td_priority));
                  return (td);
          }
  
          return (NULL);
  }
  
  /*
   * Initialize a thread queue.
   */
  static void
  tdq_setup(struct tdq *tdq, int id)
  {
  
          if (bootverbose)
                  printf("ULE: setup cpu %d\n", id);
          runq_init(&tdq->tdq_realtime);
          runq_init(&tdq->tdq_timeshare);
          runq_init(&tdq->tdq_idle);
          tdq->tdq_id = id;
          snprintf(tdq->tdq_name, sizeof(tdq->tdq_name),
              "sched lock %d", (int)TDQ_ID(tdq));
          mtx_init(&tdq->tdq_lock, tdq->tdq_name, "sched lock", MTX_SPIN);
  #ifdef KTR
          snprintf(tdq->tdq_loadname, sizeof(tdq->tdq_loadname),
              "CPU %d load", (int)TDQ_ID(tdq));
  #endif
  }
  
  #ifdef SMP
  static void
  sched_setup_smp(void)
  {
          struct tdq *tdq;
          int i;
  
          cpu_top = smp_topo();
          CPU_FOREACH(i) {
                  tdq = DPCPU_ID_PTR(i, tdq);
                  tdq_setup(tdq, i);
                  tdq->tdq_cg = smp_topo_find(cpu_top, i);
                  if (tdq->tdq_cg == NULL)
                          panic("Can't find cpu group for %d\n", i);
                  DPCPU_ID_SET(i, randomval, i * 69069 + 5);
          }
          PCPU_SET(sched, DPCPU_PTR(tdq));
          balance_tdq = TDQ_SELF();
  }
  #endif
  
  /*
   * Setup the thread queues and initialize the topology based on MD
   * information.
   */
  static void
  sched_setup(void *dummy)
  {
          struct tdq *tdq;
  
  #ifdef SMP
          sched_setup_smp();
  #else
          tdq_setup(TDQ_SELF(), 0);
  #endif
          tdq = TDQ_SELF();
  
          /* Add thread0's load since it's running. */
          TDQ_LOCK(tdq);
          thread0.td_lock = TDQ_LOCKPTR(tdq);
          tdq_load_add(tdq, &thread0);
          tdq->tdq_curthread = &thread0;
          tdq->tdq_lowpri = thread0.td_priority;
          TDQ_UNLOCK(tdq);
  }
  
  /*
   * This routine determines time constants after stathz and hz are setup.
   */
  /* ARGSUSED */
  static void
  sched_initticks(void *dummy)
  {
          int incr;
  
          realstathz = stathz ? stathz : hz;
          sched_slice = realstathz / SCHED_SLICE_DEFAULT_DIVISOR;
          sched_slice_min = sched_slice / SCHED_SLICE_MIN_DIVISOR;
          hogticks = imax(1, (2 * hz * sched_slice + realstathz / 2) /
              realstathz);
  
          /*
           * tickincr is shifted out by 10 to avoid rounding errors due to
           * hz not being evenly divisible by stathz on all platforms.
           */
          incr = (hz << SCHED_TICK_SHIFT) / realstathz;
          /*
           * This does not work for values of stathz that are more than
           * 1 << SCHED_TICK_SHIFT * hz.  In practice this does not happen.
           */
          if (incr == 0)
                  incr = 1;
          tickincr = incr;
  #ifdef SMP
          /*
           * Set the default balance interval now that we know
           * what realstathz is.
           */
          balance_interval = realstathz;
          balance_ticks = balance_interval;
          affinity = SCHED_AFFINITY_DEFAULT;
  #endif
          if (sched_idlespinthresh < 0)
                  sched_idlespinthresh = 2 * max(10000, 6 * hz) / realstathz;
  }
  
  /*
   * This is the core of the interactivity algorithm.  Determines a score based
   * on past behavior.  It is the ratio of sleep time to run time scaled to
   * a [0, 100] integer.  This is the voluntary sleep time of a process, which
   * differs from the cpu usage because it does not account for time spent
   * waiting on a run-queue.  Would be prettier if we had floating point.
   *
   * When a thread's sleep time is greater than its run time the
   * calculation is:
   *
   *                           scaling factor
   * interactivity score =  ---------------------
   *                        sleep time / run time
   *
   *
   * When a thread's run time is greater than its sleep time the
   * calculation is:
   *
   *                                                 scaling factor
   * interactivity score = 2 * scaling factor  -  ---------------------
   *                                              run time / sleep time
   */
  static int
  sched_interact_score(struct thread *td)
  {
          struct td_sched *ts;
          int div;
  
          ts = td_get_sched(td);
          /*
           * The score is only needed if this is likely to be an interactive
           * task.  Don't go through the expense of computing it if there's
           * no chance.
           */
          if (sched_interact <= SCHED_INTERACT_HALF &&
                  ts->ts_runtime >= ts->ts_slptime)
                          return (SCHED_INTERACT_HALF);
  
          if (ts->ts_runtime > ts->ts_slptime) {
                  div = max(1, ts->ts_runtime / SCHED_INTERACT_HALF);
                  return (SCHED_INTERACT_HALF +
                      (SCHED_INTERACT_HALF - (ts->ts_slptime / div)));
          }
          if (ts->ts_slptime > ts->ts_runtime) {
                  div = max(1, ts->ts_slptime / SCHED_INTERACT_HALF);
                  return (ts->ts_runtime / div);
          }
          /* runtime == slptime */
          if (ts->ts_runtime)
                  return (SCHED_INTERACT_HALF);
  
          /*
           * This can happen if slptime and runtime are 0.
           */
          return (0);
  
  }
  
  /*
   * Scale the scheduling priority according to the "interactivity" of this
   * process.
   */
  static void
  sched_priority(struct thread *td)
  {
          u_int pri, score;
  
          if (PRI_BASE(td->td_pri_class) != PRI_TIMESHARE)
                  return;
          /*
           * If the score is interactive we place the thread in the realtime
           * queue with a priority that is less than kernel and interrupt
           * priorities.  These threads are not subject to nice restrictions.
           *
           * Scores greater than this are placed on the normal timeshare queue
           * where the priority is partially decided by the most recent cpu
           * utilization and the rest is decided by nice value.
           *
           * The nice value of the process has a linear effect on the calculated
           * score.  Negative nice values make it easier for a thread to be
           * considered interactive.
           */
          score = imax(0, sched_interact_score(td) + td->td_proc->p_nice);
          if (score < sched_interact) {
                  pri = PRI_MIN_INTERACT;
                  pri += (PRI_MAX_INTERACT - PRI_MIN_INTERACT + 1) * score /
                      sched_interact;
                  KASSERT(pri >= PRI_MIN_INTERACT && pri <= PRI_MAX_INTERACT,
                      ("sched_priority: invalid interactive priority %u score %u",
                      pri, score));
          } else {
                  pri = SCHED_PRI_MIN;
                  if (td_get_sched(td)->ts_ticks)
                          pri += min(SCHED_PRI_TICKS(td_get_sched(td)),
                              SCHED_PRI_RANGE - 1);
                  pri += SCHED_PRI_NICE(td->td_proc->p_nice);
                  KASSERT(pri >= PRI_MIN_BATCH && pri <= PRI_MAX_BATCH,
                      ("sched_priority: invalid priority %u: nice %d, "
                      "ticks %d ftick %d ltick %d tick pri %d",
                      pri, td->td_proc->p_nice, td_get_sched(td)->ts_ticks,
                      td_get_sched(td)->ts_ftick, td_get_sched(td)->ts_ltick,
                      SCHED_PRI_TICKS(td_get_sched(td))));
          }
          sched_user_prio(td, pri);
  
          return;
  }
  
  /*
   * This routine enforces a maximum limit on the amount of scheduling history
   * kept.  It is called after either the slptime or runtime is adjusted.  This
   * function is ugly due to integer math.
   */
  static void
  sched_interact_update(struct thread *td)
  {
          struct td_sched *ts;
          u_int sum;
  
          ts = td_get_sched(td);
          sum = ts->ts_runtime + ts->ts_slptime;
          if (sum < SCHED_SLP_RUN_MAX)
                  return;
          /*
           * This only happens from two places:
           * 1) We have added an unusual amount of run time from fork_exit.
           * 2) We have added an unusual amount of sleep time from sched_sleep().
           */
          if (sum > SCHED_SLP_RUN_MAX * 2) {
                  if (ts->ts_runtime > ts->ts_slptime) {
                          ts->ts_runtime = SCHED_SLP_RUN_MAX;
                          ts->ts_slptime = 1;
                  } else {
                          ts->ts_slptime = SCHED_SLP_RUN_MAX;
                          ts->ts_runtime = 1;
                  }
                  return;
          }
          /*
           * If we have exceeded by more than 1/5th then the algorithm below
           * will not bring us back into range.  Dividing by two here forces
           * us into the range of [4/5 * SCHED_INTERACT_MAX, SCHED_INTERACT_MAX]
           */
          if (sum > (SCHED_SLP_RUN_MAX / 5) * 6) {
                  ts->ts_runtime /= 2;
                  ts->ts_slptime /= 2;
                  return;
          }
          ts->ts_runtime = (ts->ts_runtime / 5) * 4;
          ts->ts_slptime = (ts->ts_slptime / 5) * 4;
  }
  
  /*
   * Scale back the interactivity history when a child thread is created.  The
   * history is inherited from the parent but the thread may behave totally
   * differently.  For example, a shell spawning a compiler process.  We want
   * to learn that the compiler is behaving badly very quickly.
   */
  static void
  sched_interact_fork(struct thread *td)
  {
          struct td_sched *ts;
          int ratio;
          int sum;
  
          ts = td_get_sched(td);
          sum = ts->ts_runtime + ts->ts_slptime;
          if (sum > SCHED_SLP_RUN_FORK) {
                  ratio = sum / SCHED_SLP_RUN_FORK;
                  ts->ts_runtime /= ratio;
                  ts->ts_slptime /= ratio;
          }
  }
  
  /*
   * Called from proc0_init() to setup the scheduler fields.
   */
  void
  schedinit(void)
  {
          struct td_sched *ts0;
  
          /*
           * Set up the scheduler specific parts of thread0.
           */
          ts0 = td_get_sched(&thread0);
          ts0->ts_ltick = ticks;
          ts0->ts_ftick = ticks;
          ts0->ts_slice = 0;
          ts0->ts_cpu = curcpu;   /* set valid CPU number */
  }
  
  /*
   * schedinit_ap() is needed prior to calling sched_throw(NULL) to ensure that
   * the pcpu requirements are met for any calls in the period between curthread
   * initialization and sched_throw().  One can safely add threads to the queue
   * before sched_throw(), for instance, as long as the thread lock is setup
   * correctly.
   *
   * TDQ_SELF() relies on the below sched pcpu setting; it may be used only
   * after schedinit_ap().
   */
  void
  schedinit_ap(void)
  {
  
  #ifdef SMP
          PCPU_SET(sched, DPCPU_PTR(tdq));
  #endif
          PCPU_GET(idlethread)->td_lock = TDQ_LOCKPTR(TDQ_SELF());
  }
  
  /*
   * This is only somewhat accurate since given many processes of the same
   * priority they will switch when their slices run out, which will be
   * at most sched_slice stathz ticks.
   */
  int
  sched_rr_interval(void)
  {
  
          /* Convert sched_slice from stathz to hz. */
          return (imax(1, (sched_slice * hz + realstathz / 2) / realstathz));
  }
  
  /*
   * Update the percent cpu tracking information when it is requested or
   * the total history exceeds the maximum.  We keep a sliding history of
   * tick counts that slowly decays.  This is less precise than the 4BSD
   * mechanism since it happens with less regular and frequent events.
   */
  static void
  sched_pctcpu_update(struct td_sched *ts, int run)
  {
          int t = ticks;
  
          /*
           * The signed difference may be negative if the thread hasn't run for
           * over half of the ticks rollover period.
           */
          if ((u_int)(t - ts->ts_ltick) >= SCHED_TICK_TARG) {
                  ts->ts_ticks = 0;
                  ts->ts_ftick = t - SCHED_TICK_TARG;
          } else if (t - ts->ts_ftick >= SCHED_TICK_MAX) {
                  ts->ts_ticks = (ts->ts_ticks / (ts->ts_ltick - ts->ts_ftick)) *
                      (ts->ts_ltick - (t - SCHED_TICK_TARG));
                  ts->ts_ftick = t - SCHED_TICK_TARG;
          }
          if (run)
                  ts->ts_ticks += (t - ts->ts_ltick) << SCHED_TICK_SHIFT;
          ts->ts_ltick = t;
  }
  
  /*
   * Adjust the priority of a thread.  Move it to the appropriate run-queue
   * if necessary.  This is the back-end for several priority related
   * functions.
   */
  static void
  sched_thread_priority(struct thread *td, u_char prio)
  {
          struct tdq *tdq;
          int oldpri;
  
          KTR_POINT3(KTR_SCHED, "thread", sched_tdname(td), "prio",
              "prio:%d", td->td_priority, "new prio:%d", prio,
              KTR_ATTR_LINKED, sched_tdname(curthread));
          SDT_PROBE3(sched, , , change__pri, td, td->td_proc, prio);
          if (td != curthread && prio < td->td_priority) {
                  KTR_POINT3(KTR_SCHED, "thread", sched_tdname(curthread),
                      "lend prio", "prio:%d", td->td_priority, "new prio:%d",
                      prio, KTR_ATTR_LINKED, sched_tdname(td));
                  SDT_PROBE4(sched, , , lend__pri, td, td->td_proc, prio, 
                      curthread);
          } 
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          if (td->td_priority == prio)
                  return;
          /*
           * If the priority has been elevated due to priority
           * propagation, we may have to move ourselves to a new
           * queue.  This could be optimized to not re-add in some
           * cases.
           */
          if (TD_ON_RUNQ(td) && prio < td->td_priority) {
                  sched_rem(td);
                  td->td_priority = prio;
                  sched_add(td, SRQ_BORROWING | SRQ_HOLDTD);
                  return;
          }
          /*
           * If the thread is currently running we may have to adjust the lowpri
           * information so other cpus are aware of our current priority.
           */
          if (TD_IS_RUNNING(td)) {
                  tdq = TDQ_CPU(td_get_sched(td)->ts_cpu);
                  oldpri = td->td_priority;
                  td->td_priority = prio;
                  if (prio < tdq->tdq_lowpri)
                          tdq->tdq_lowpri = prio;
                  else if (tdq->tdq_lowpri == oldpri)
                          tdq_setlowpri(tdq, td);
                  return;
          }
          td->td_priority = prio;
  }
  
  /*
   * Update a thread's priority when it is lent another thread's
   * priority.
   */
  void
  sched_lend_prio(struct thread *td, u_char prio)
  {
  
          td->td_flags |= TDF_BORROWING;
          sched_thread_priority(td, prio);
  }
  
  /*
   * Restore a thread's priority when priority propagation is
   * over.  The prio argument is the minimum priority the thread
   * needs to have to satisfy other possible priority lending
   * requests.  If the thread's regular priority is less
   * important than prio, the thread will keep a priority boost
   * of prio.
   */
  void
  sched_unlend_prio(struct thread *td, u_char prio)
  {
          u_char base_pri;
  
          if (td->td_base_pri >= PRI_MIN_TIMESHARE &&
              td->td_base_pri <= PRI_MAX_TIMESHARE)
                  base_pri = td->td_user_pri;
          else
                  base_pri = td->td_base_pri;
          if (prio >= base_pri) {
                  td->td_flags &= ~TDF_BORROWING;
                  sched_thread_priority(td, base_pri);
          } else
                  sched_lend_prio(td, prio);
  }
  
  /*
   * Standard entry for setting the priority to an absolute value.
   */
  void
  sched_prio(struct thread *td, u_char prio)
  {
          u_char oldprio;
  
          /* First, update the base priority. */
          td->td_base_pri = prio;
  
          /*
           * If the thread is borrowing another thread's priority, don't
           * ever lower the priority.
           */
          if (td->td_flags & TDF_BORROWING && td->td_priority < prio)
                  return;
  
          /* Change the real priority. */
          oldprio = td->td_priority;
          sched_thread_priority(td, prio);
  
          /*
           * If the thread is on a turnstile, then let the turnstile update
           * its state.
           */
          if (TD_ON_LOCK(td) && oldprio != prio)
                  turnstile_adjust(td, oldprio);
  }
  
  /*
   * Set the base interrupt thread priority.
   */
  void
  sched_ithread_prio(struct thread *td, u_char prio)
  {
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          MPASS(td->td_pri_class == PRI_ITHD);
          td->td_base_ithread_pri = prio;
          sched_prio(td, prio);
  }
  
  /*
   * Set the base user priority, does not effect current running priority.
   */
  void
  sched_user_prio(struct thread *td, u_char prio)
  {
  
          td->td_base_user_pri = prio;
          if (td->td_lend_user_pri <= prio)
                  return;
          td->td_user_pri = prio;
  }
  
  void
  sched_lend_user_prio(struct thread *td, u_char prio)
  {
  
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          td->td_lend_user_pri = prio;
          td->td_user_pri = min(prio, td->td_base_user_pri);
          if (td->td_priority > td->td_user_pri)
                  sched_prio(td, td->td_user_pri);
          else if (td->td_priority != td->td_user_pri)
                  ast_sched_locked(td, TDA_SCHED);
  }
  
  /*
   * Like the above but first check if there is anything to do.
   */
  void
  sched_lend_user_prio_cond(struct thread *td, u_char prio)
  {
  
          if (td->td_lend_user_pri != prio)
                  goto lend;
          if (td->td_user_pri != min(prio, td->td_base_user_pri))
                  goto lend;
          if (td->td_priority != td->td_user_pri)
                  goto lend;
          return;
  
  lend:
          thread_lock(td);
          sched_lend_user_prio(td, prio);
          thread_unlock(td);
  }
  
  #ifdef SMP
  /*
   * This tdq is about to idle.  Try to steal a thread from another CPU before
   * choosing the idle thread.
   */
  static void
  tdq_trysteal(struct tdq *tdq)
  {
          struct cpu_group *cg, *parent;
          struct tdq *steal;
          cpuset_t mask;
          int cpu, i, goup;
  
          if (smp_started == 0 || steal_idle == 0 || trysteal_limit == 0 ||
              tdq->tdq_cg == NULL)
                  return;
          CPU_FILL(&mask);
          CPU_CLR(PCPU_GET(cpuid), &mask);
          /* We don't want to be preempted while we're iterating. */
          spinlock_enter();
          TDQ_UNLOCK(tdq);
          for (i = 1, cg = tdq->tdq_cg, goup = 0; ; ) {
                  cpu = sched_highest(cg, &mask, steal_thresh, 1);
                  /*
                   * If a thread was added while interrupts were disabled don't
                   * steal one here.
                   */
                  if (TDQ_LOAD(tdq) > 0) {
                          TDQ_LOCK(tdq);
                          break;
                  }
  
                  /*
                   * We found no CPU to steal from in this group.  Escalate to
                   * the parent and repeat.  But if parent has only two children
                   * groups we can avoid searching this group again by searching
                   * the other one specifically and then escalating two levels.
                   */
                  if (cpu == -1) {
                          if (goup) {
                                  cg = cg->cg_parent;
                                  goup = 0;
                          }
                          if (++i > trysteal_limit) {
                                  TDQ_LOCK(tdq);
                                  break;
                          }
                          parent = cg->cg_parent;
                          if (parent == NULL) {
                                  TDQ_LOCK(tdq);
                                  break;
                          }
                          if (parent->cg_children == 2) {
                                  if (cg == &parent->cg_child[0])
                                          cg = &parent->cg_child[1];
                                  else
                                          cg = &parent->cg_child[0];
                                  goup = 1;
                          } else
                                  cg = parent;
                          continue;
                  }
                  steal = TDQ_CPU(cpu);
                  /*
                   * The data returned by sched_highest() is stale and
                   * the chosen CPU no longer has an eligible thread.
                   * At this point unconditionally exit the loop to bound
                   * the time spent in the critcal section.
                   */
                  if (TDQ_LOAD(steal) < steal_thresh ||
                      TDQ_TRANSFERABLE(steal) == 0)
                          continue;
                  /*
                   * Try to lock both queues. If we are assigned a thread while
                   * waited for the lock, switch to it now instead of stealing.
                   * If we can't get the lock, then somebody likely got there
                   * first.
                   */
                  TDQ_LOCK(tdq);
                  if (tdq->tdq_load > 0)
                          break;
                  if (TDQ_TRYLOCK_FLAGS(steal, MTX_DUPOK) == 0)
                          break;
                  /*
                   * The data returned by sched_highest() is stale and
                   * the chosen CPU no longer has an eligible thread.
                   */
                  if (TDQ_LOAD(steal) < steal_thresh ||
                      TDQ_TRANSFERABLE(steal) == 0) {
                          TDQ_UNLOCK(steal);
                          break;
                  }
                  /*
                   * If we fail to acquire one due to affinity restrictions,
                   * bail out and let the idle thread to a more complete search
                   * outside of a critical section.
                   */
                  if (tdq_move(steal, tdq) == -1) {
                          TDQ_UNLOCK(steal);
                          break;
                  }
                  TDQ_UNLOCK(steal);
                  break;
          }
          spinlock_exit();
  }
  #endif
  
  /*
   * Handle migration from sched_switch().  This happens only for
   * cpu binding.
   */
  static struct mtx *
  sched_switch_migrate(struct tdq *tdq, struct thread *td, int flags)
  {
          struct tdq *tdn;
  #ifdef SMP
          int lowpri;
  #endif
  
          KASSERT(THREAD_CAN_MIGRATE(td) ||
              (td_get_sched(td)->ts_flags & TSF_BOUND) != 0,
              ("Thread %p shouldn't migrate", td));
          KASSERT(!CPU_ABSENT(td_get_sched(td)->ts_cpu), ("sched_switch_migrate: "
              "thread %s queued on absent CPU %d.", td->td_name,
              td_get_sched(td)->ts_cpu));
          tdn = TDQ_CPU(td_get_sched(td)->ts_cpu);
  #ifdef SMP
          tdq_load_rem(tdq, td);
          /*
           * Do the lock dance required to avoid LOR.  We have an 
           * extra spinlock nesting from sched_switch() which will
           * prevent preemption while we're holding neither run-queue lock.
           */
          TDQ_UNLOCK(tdq);
          TDQ_LOCK(tdn);
          lowpri = tdq_add(tdn, td, flags);
          tdq_notify(tdn, lowpri);
          TDQ_UNLOCK(tdn);
          TDQ_LOCK(tdq);
  #endif
          return (TDQ_LOCKPTR(tdn));
  }
  
  /*
   * thread_lock_unblock() that does not assume td_lock is blocked.
   */
  static inline void
  thread_unblock_switch(struct thread *td, struct mtx *mtx)
  {
          atomic_store_rel_ptr((volatile uintptr_t *)&td->td_lock,
              (uintptr_t)mtx);
  }
  
  /*
   * Switch threads.  This function has to handle threads coming in while
   * blocked for some reason, running, or idle.  It also must deal with
   * migrating a thread from one queue to another as running threads may
   * be assigned elsewhere via binding.
   */
  void
  sched_switch(struct thread *td, int flags)
  {
          struct thread *newtd;
          struct tdq *tdq;
          struct td_sched *ts;
          struct mtx *mtx;
          int srqflag;
          int cpuid, preempted;
  #ifdef SMP
          int pickcpu;
  #endif
  
          THREAD_LOCK_ASSERT(td, MA_OWNED);
  
          cpuid = PCPU_GET(cpuid);
          tdq = TDQ_SELF();
          ts = td_get_sched(td);
          sched_pctcpu_update(ts, 1);
  #ifdef SMP
          pickcpu = (td->td_flags & TDF_PICKCPU) != 0;
          if (pickcpu)
                  ts->ts_rltick = ticks - affinity * MAX_CACHE_LEVELS;
          else
                  ts->ts_rltick = ticks;
  #endif
          td->td_lastcpu = td->td_oncpu;
          preempted = (td->td_flags & TDF_SLICEEND) == 0 &&
              (flags & SW_PREEMPT) != 0;
          td->td_flags &= ~(TDF_PICKCPU | TDF_SLICEEND);
          ast_unsched_locked(td, TDA_SCHED);
          td->td_owepreempt = 0;
          atomic_store_char(&tdq->tdq_owepreempt, 0);
          if (!TD_IS_IDLETHREAD(td))
                  TDQ_SWITCHCNT_INC(tdq);
  
          /*
           * Always block the thread lock so we can drop the tdq lock early.
           */
          mtx = thread_lock_block(td);
          spinlock_enter();
          if (TD_IS_IDLETHREAD(td)) {
                  MPASS(mtx == TDQ_LOCKPTR(tdq));
                  TD_SET_CAN_RUN(td);
          } else if (TD_IS_RUNNING(td)) {
                  MPASS(mtx == TDQ_LOCKPTR(tdq));
                  srqflag = preempted ?
                      SRQ_OURSELF|SRQ_YIELDING|SRQ_PREEMPTED :
                      SRQ_OURSELF|SRQ_YIELDING;
  #ifdef SMP
                  if (THREAD_CAN_MIGRATE(td) && (!THREAD_CAN_SCHED(td, ts->ts_cpu)
                      || pickcpu))
                          ts->ts_cpu = sched_pickcpu(td, 0);
  #endif
                  if (ts->ts_cpu == cpuid)
                          tdq_runq_add(tdq, td, srqflag);
                  else
                          mtx = sched_switch_migrate(tdq, td, srqflag);
          } else {
                  /* This thread must be going to sleep. */
                  if (mtx != TDQ_LOCKPTR(tdq)) {
                          mtx_unlock_spin(mtx);
                          TDQ_LOCK(tdq);
                  }
                  tdq_load_rem(tdq, td);
  #ifdef SMP
                  if (tdq->tdq_load == 0)
                          tdq_trysteal(tdq);
  #endif
          }
  
  #if (KTR_COMPILE & KTR_SCHED) != 0
          if (TD_IS_IDLETHREAD(td))
                  KTR_STATE1(KTR_SCHED, "thread", sched_tdname(td), "idle",
                      "prio:%d", td->td_priority);
          else
                  KTR_STATE3(KTR_SCHED, "thread", sched_tdname(td), KTDSTATE(td),
                      "prio:%d", td->td_priority, "wmesg:\"%s\"", td->td_wmesg,
                      "lockname:\"%s\"", td->td_lockname);
  #endif
  
          /*
           * We enter here with the thread blocked and assigned to the
           * appropriate cpu run-queue or sleep-queue and with the current
           * thread-queue locked.
           */
          TDQ_LOCK_ASSERT(tdq, MA_OWNED | MA_NOTRECURSED);
          MPASS(td == tdq->tdq_curthread);
          newtd = choosethread();
          sched_pctcpu_update(td_get_sched(newtd), 0);
          TDQ_UNLOCK(tdq);
  
          /*
           * Call the MD code to switch contexts if necessary.
           */
          if (td != newtd) {
  #ifdef  HWPMC_HOOKS
                  if (PMC_PROC_IS_USING_PMCS(td->td_proc))
                          PMC_SWITCH_CONTEXT(td, PMC_FN_CSW_OUT);
  #endif
                  SDT_PROBE2(sched, , , off__cpu, newtd, newtd->td_proc);
  
  #ifdef KDTRACE_HOOKS
                  /*
                   * If DTrace has set the active vtime enum to anything
                   * other than INACTIVE (0), then it should have set the
                   * function to call.
                   */
                  if (dtrace_vtime_active)
                          (*dtrace_vtime_switch_func)(newtd);
  #endif
                  td->td_oncpu = NOCPU;
                  cpu_switch(td, newtd, mtx);
                  cpuid = td->td_oncpu = PCPU_GET(cpuid);
  
                  SDT_PROBE0(sched, , , on__cpu);
  #ifdef  HWPMC_HOOKS
                  if (PMC_PROC_IS_USING_PMCS(td->td_proc))
                          PMC_SWITCH_CONTEXT(td, PMC_FN_CSW_IN);
  #endif
          } else {
                  thread_unblock_switch(td, mtx);
                  SDT_PROBE0(sched, , , remain__cpu);
          }
          KASSERT(curthread->td_md.md_spinlock_count == 1,
              ("invalid count %d", curthread->td_md.md_spinlock_count));
  
          KTR_STATE1(KTR_SCHED, "thread", sched_tdname(td), "running",
              "prio:%d", td->td_priority);
  }
  
  /*
   * Adjust thread priorities as a result of a nice request.
   */
  void
  sched_nice(struct proc *p, int nice)
  {
          struct thread *td;
  
          PROC_LOCK_ASSERT(p, MA_OWNED);
  
          p->p_nice = nice;
          FOREACH_THREAD_IN_PROC(p, td) {
                  thread_lock(td);
                  sched_priority(td);
                  sched_prio(td, td->td_base_user_pri);
                  thread_unlock(td);
          }
  }
  
  /*
   * Record the sleep time for the interactivity scorer.
   */
  void
  sched_sleep(struct thread *td, int prio)
  {
  
          THREAD_LOCK_ASSERT(td, MA_OWNED);
  
          td->td_slptick = ticks;
          if (TD_IS_SUSPENDED(td) || prio >= PSOCK)
                  td->td_flags |= TDF_CANSWAP;
          if (PRI_BASE(td->td_pri_class) != PRI_TIMESHARE)
                  return;
          if (static_boost == 1 && prio)
                  sched_prio(td, prio);
          else if (static_boost && td->td_priority > static_boost)
                  sched_prio(td, static_boost);
  }
  
  /*
   * Schedule a thread to resume execution and record how long it voluntarily
   * slept.  We also update the pctcpu, interactivity, and priority.
   *
   * Requires the thread lock on entry, drops on exit.
   */
  void
  sched_wakeup(struct thread *td, int srqflags)
  {
          struct td_sched *ts;
          int slptick;
  
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          ts = td_get_sched(td);
          td->td_flags &= ~TDF_CANSWAP;
  
          /*
           * If we slept for more than a tick update our interactivity and
           * priority.
           */
          slptick = td->td_slptick;
          td->td_slptick = 0;
          if (slptick && slptick != ticks) {
                  ts->ts_slptime += (ticks - slptick) << SCHED_TICK_SHIFT;
                  sched_interact_update(td);
                  sched_pctcpu_update(ts, 0);
          }
  
          /*
           * When resuming an idle ithread, restore its base ithread
           * priority.
           */
          if (PRI_BASE(td->td_pri_class) == PRI_ITHD &&
              td->td_priority != td->td_base_ithread_pri)
                  sched_prio(td, td->td_base_ithread_pri);
  
          /*
           * Reset the slice value since we slept and advanced the round-robin.
           */
          ts->ts_slice = 0;
          sched_add(td, SRQ_BORING | srqflags);
  }
  
  /*
   * Penalize the parent for creating a new child and initialize the child's
   * priority.
   */
  void
  sched_fork(struct thread *td, struct thread *child)
  {
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          sched_pctcpu_update(td_get_sched(td), 1);
          sched_fork_thread(td, child);
          /*
           * Penalize the parent and child for forking.
           */
          sched_interact_fork(child);
          sched_priority(child);
          td_get_sched(td)->ts_runtime += tickincr;
          sched_interact_update(td);
          sched_priority(td);
  }
  
  /*
   * Fork a new thread, may be within the same process.
   */
  void
  sched_fork_thread(struct thread *td, struct thread *child)
  {
          struct td_sched *ts;
          struct td_sched *ts2;
          struct tdq *tdq;
  
          tdq = TDQ_SELF();
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          /*
           * Initialize child.
           */
          ts = td_get_sched(td);
          ts2 = td_get_sched(child);
          child->td_oncpu = NOCPU;
          child->td_lastcpu = NOCPU;
          child->td_lock = TDQ_LOCKPTR(tdq);
          child->td_cpuset = cpuset_ref(td->td_cpuset);
          child->td_domain.dr_policy = td->td_cpuset->cs_domain;
          ts2->ts_cpu = ts->ts_cpu;
          ts2->ts_flags = 0;
          /*
           * Grab our parents cpu estimation information.
           */
          ts2->ts_ticks = ts->ts_ticks;
          ts2->ts_ltick = ts->ts_ltick;
          ts2->ts_ftick = ts->ts_ftick;
          /*
           * Do not inherit any borrowed priority from the parent.
           */
          child->td_priority = child->td_base_pri;
          /*
           * And update interactivity score.
           */
          ts2->ts_slptime = ts->ts_slptime;
          ts2->ts_runtime = ts->ts_runtime;
          /* Attempt to quickly learn interactivity. */
          ts2->ts_slice = tdq_slice(tdq) - sched_slice_min;
  #ifdef KTR
          bzero(ts2->ts_name, sizeof(ts2->ts_name));
  #endif
  }
  
  /*
   * Adjust the priority class of a thread.
   */
  void
  sched_class(struct thread *td, int class)
  {
  
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          if (td->td_pri_class == class)
                  return;
          td->td_pri_class = class;
  }
  
  /*
   * Return some of the child's priority and interactivity to the parent.
   */
  void
  sched_exit(struct proc *p, struct thread *child)
  {
          struct thread *td;
  
          KTR_STATE1(KTR_SCHED, "thread", sched_tdname(child), "proc exit",
              "prio:%d", child->td_priority);
          PROC_LOCK_ASSERT(p, MA_OWNED);
          td = FIRST_THREAD_IN_PROC(p);
          sched_exit_thread(td, child);
  }
  
  /*
   * Penalize another thread for the time spent on this one.  This helps to
   * worsen the priority and interactivity of processes which schedule batch
   * jobs such as make.  This has little effect on the make process itself but
   * causes new processes spawned by it to receive worse scores immediately.
   */
  void
  sched_exit_thread(struct thread *td, struct thread *child)
  {
  
          KTR_STATE1(KTR_SCHED, "thread", sched_tdname(child), "thread exit",
              "prio:%d", child->td_priority);
          /*
           * Give the child's runtime to the parent without returning the
           * sleep time as a penalty to the parent.  This causes shells that
           * launch expensive things to mark their children as expensive.
           */
          thread_lock(td);
          td_get_sched(td)->ts_runtime += td_get_sched(child)->ts_runtime;
          sched_interact_update(td);
          sched_priority(td);
          thread_unlock(td);
  }
  
  void
  sched_preempt(struct thread *td)
  {
          struct tdq *tdq;
          int flags;
  
          SDT_PROBE2(sched, , , surrender, td, td->td_proc);
  
          thread_lock(td);
          tdq = TDQ_SELF();
          TDQ_LOCK_ASSERT(tdq, MA_OWNED);
          if (td->td_priority > tdq->tdq_lowpri) {
                  if (td->td_critnest == 1) {
                          flags = SW_INVOL | SW_PREEMPT;
                          flags |= TD_IS_IDLETHREAD(td) ? SWT_REMOTEWAKEIDLE :
                              SWT_REMOTEPREEMPT;
                          mi_switch(flags);
                          /* Switch dropped thread lock. */
                          return;
                  }
                  td->td_owepreempt = 1;
          } else {
                  tdq->tdq_owepreempt = 0;
          }
          thread_unlock(td);
  }
  
  /*
   * Fix priorities on return to user-space.  Priorities may be elevated due
   * to static priorities in msleep() or similar.
   */
  void
  sched_userret_slowpath(struct thread *td)
  {
  
          thread_lock(td);
          td->td_priority = td->td_user_pri;
          td->td_base_pri = td->td_user_pri;
          tdq_setlowpri(TDQ_SELF(), td);
          thread_unlock(td);
  }
  
  SCHED_STAT_DEFINE(ithread_demotions, "Interrupt thread priority demotions");
  SCHED_STAT_DEFINE(ithread_preemptions,
      "Interrupt thread preemptions due to time-sharing");
  
  /*
   * Return time slice for a given thread.  For ithreads this is
   * sched_slice.  For other threads it is tdq_slice(tdq).
   */
  static inline int
  td_slice(struct thread *td, struct tdq *tdq)
  {
          if (PRI_BASE(td->td_pri_class) == PRI_ITHD)
                  return (sched_slice);
          return (tdq_slice(tdq));
  }
  
  /*
   * Handle a stathz tick.  This is really only relevant for timeshare
   * and interrupt threads.
   */
  void
  sched_clock(struct thread *td, int cnt)
  {
          struct tdq *tdq;
          struct td_sched *ts;
  
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          tdq = TDQ_SELF();
  #ifdef SMP
          /*
           * We run the long term load balancer infrequently on the first cpu.
           */
          if (balance_tdq == tdq && smp_started != 0 && rebalance != 0 &&
              balance_ticks != 0) {
                  balance_ticks -= cnt;
                  if (balance_ticks <= 0)
                          sched_balance();
          }
  #endif
          /*
           * Save the old switch count so we have a record of the last ticks
           * activity.   Initialize the new switch count based on our load.
           * If there is some activity seed it to reflect that.
           */
          tdq->tdq_oldswitchcnt = tdq->tdq_switchcnt;
          tdq->tdq_switchcnt = tdq->tdq_load;
  
          /*
           * Advance the insert index once for each tick to ensure that all
           * threads get a chance to run.
           */
          if (tdq->tdq_idx == tdq->tdq_ridx) {
                  tdq->tdq_idx = (tdq->tdq_idx + 1) % RQ_NQS;
                  if (TAILQ_EMPTY(&tdq->tdq_timeshare.rq_queues[tdq->tdq_ridx]))
                          tdq->tdq_ridx = tdq->tdq_idx;
          }
          ts = td_get_sched(td);
          sched_pctcpu_update(ts, 1);
          if ((td->td_pri_class & PRI_FIFO_BIT) || TD_IS_IDLETHREAD(td))
                  return;
  
          if (PRI_BASE(td->td_pri_class) == PRI_TIMESHARE) {
                  /*
                   * We used a tick; charge it to the thread so
                   * that we can compute our interactivity.
                   */
                  td_get_sched(td)->ts_runtime += tickincr * cnt;
                  sched_interact_update(td);
                  sched_priority(td);
          }
  
          /*
           * Force a context switch if the current thread has used up a full
           * time slice (default is 100ms).
           */
          ts->ts_slice += cnt;
          if (ts->ts_slice >= td_slice(td, tdq)) {
                  ts->ts_slice = 0;
  
                  /*
                   * If an ithread uses a full quantum, demote its
                   * priority and preempt it.
                   */
                  if (PRI_BASE(td->td_pri_class) == PRI_ITHD) {
                          SCHED_STAT_INC(ithread_preemptions);
                          td->td_owepreempt = 1;
                          if (td->td_base_pri + RQ_PPQ < PRI_MAX_ITHD) {
                                  SCHED_STAT_INC(ithread_demotions);
                                  sched_prio(td, td->td_base_pri + RQ_PPQ);
                          }
                  } else {
                          ast_sched_locked(td, TDA_SCHED);
                          td->td_flags |= TDF_SLICEEND;
                  }
          }
  }
  
  u_int
  sched_estcpu(struct thread *td __unused)
  {
  
          return (0);
  }
  
  /*
   * Return whether the current CPU has runnable tasks.  Used for in-kernel
   * cooperative idle threads.
   */
  int
  sched_runnable(void)
  {
          struct tdq *tdq;
          int load;
  
          load = 1;
  
          tdq = TDQ_SELF();
          if ((curthread->td_flags & TDF_IDLETD) != 0) {
                  if (TDQ_LOAD(tdq) > 0)
                          goto out;
          } else
                  if (TDQ_LOAD(tdq) - 1 > 0)
                          goto out;
          load = 0;
  out:
          return (load);
  }
  
  /*
   * Choose the highest priority thread to run.  The thread is removed from
   * the run-queue while running however the load remains.
   */
  struct thread *
  sched_choose(void)
  {
          struct thread *td;
          struct tdq *tdq;
  
          tdq = TDQ_SELF();
          TDQ_LOCK_ASSERT(tdq, MA_OWNED);
          td = tdq_choose(tdq);
          if (td != NULL) {
                  tdq_runq_rem(tdq, td);
                  tdq->tdq_lowpri = td->td_priority;
          } else { 
                  tdq->tdq_lowpri = PRI_MAX_IDLE;
                  td = PCPU_GET(idlethread);
          }
          tdq->tdq_curthread = td;
          return (td);
  }
  
  /*
   * Set owepreempt if the currently running thread has lower priority than "pri".
   * Preemption never happens directly in ULE, we always request it once we exit a
   * critical section.
   */
  static void
  sched_setpreempt(int pri)
  {
          struct thread *ctd;
          int cpri;
  
          ctd = curthread;
          THREAD_LOCK_ASSERT(ctd, MA_OWNED);
  
          cpri = ctd->td_priority;
          if (pri < cpri)
                  ast_sched_locked(ctd, TDA_SCHED);
          if (KERNEL_PANICKED() || pri >= cpri || cold || TD_IS_INHIBITED(ctd))
                  return;
          if (!sched_shouldpreempt(pri, cpri, 0))
                  return;
          ctd->td_owepreempt = 1;
  }
  
  /*
   * Add a thread to a thread queue.  Select the appropriate runq and add the
   * thread to it.  This is the internal function called when the tdq is
   * predetermined.
   */
  static int
  tdq_add(struct tdq *tdq, struct thread *td, int flags)
  {
          int lowpri;
  
          TDQ_LOCK_ASSERT(tdq, MA_OWNED);
          THREAD_LOCK_BLOCKED_ASSERT(td, MA_OWNED);
          KASSERT((td->td_inhibitors == 0),
              ("sched_add: trying to run inhibited thread"));
          KASSERT((TD_CAN_RUN(td) || TD_IS_RUNNING(td)),
              ("sched_add: bad thread state"));
          KASSERT(td->td_flags & TDF_INMEM,
              ("sched_add: thread swapped out"));
  
          lowpri = tdq->tdq_lowpri;
          if (td->td_priority < lowpri)
                  tdq->tdq_lowpri = td->td_priority;
          tdq_runq_add(tdq, td, flags);
          tdq_load_add(tdq, td);
          return (lowpri);
  }
  
  /*
   * Select the target thread queue and add a thread to it.  Request
   * preemption or IPI a remote processor if required.
   *
   * Requires the thread lock on entry, drops on exit.
   */
  void
  sched_add(struct thread *td, int flags)
  {
          struct tdq *tdq;
  #ifdef SMP
          int cpu, lowpri;
  #endif
  
          KTR_STATE2(KTR_SCHED, "thread", sched_tdname(td), "runq add",
              "prio:%d", td->td_priority, KTR_ATTR_LINKED,
              sched_tdname(curthread));
          KTR_POINT1(KTR_SCHED, "thread", sched_tdname(curthread), "wokeup",
              KTR_ATTR_LINKED, sched_tdname(td));
          SDT_PROBE4(sched, , , enqueue, td, td->td_proc, NULL, 
              flags & SRQ_PREEMPTED);
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          /*
           * Recalculate the priority before we select the target cpu or
           * run-queue.
           */
          if (PRI_BASE(td->td_pri_class) == PRI_TIMESHARE)
                  sched_priority(td);
  #ifdef SMP
          /*
           * Pick the destination cpu and if it isn't ours transfer to the
           * target cpu.
           */
          cpu = sched_pickcpu(td, flags);
          tdq = sched_setcpu(td, cpu, flags);
          lowpri = tdq_add(tdq, td, flags);
          if (cpu != PCPU_GET(cpuid))
                  tdq_notify(tdq, lowpri);
          else if (!(flags & SRQ_YIELDING))
                  sched_setpreempt(td->td_priority);
  #else
          tdq = TDQ_SELF();
          /*
           * Now that the thread is moving to the run-queue, set the lock
           * to the scheduler's lock.
           */
          if (td->td_lock != TDQ_LOCKPTR(tdq)) {
                  TDQ_LOCK(tdq);
                  if ((flags & SRQ_HOLD) != 0)
                          td->td_lock = TDQ_LOCKPTR(tdq);
                  else
                          thread_lock_set(td, TDQ_LOCKPTR(tdq));
          }
          (void)tdq_add(tdq, td, flags);
          if (!(flags & SRQ_YIELDING))
                  sched_setpreempt(td->td_priority);
  #endif
          if (!(flags & SRQ_HOLDTD))
                  thread_unlock(td);
  }
  
  /*
   * Remove a thread from a run-queue without running it.  This is used
   * when we're stealing a thread from a remote queue.  Otherwise all threads
   * exit by calling sched_exit_thread() and sched_throw() themselves.
   */
  void
  sched_rem(struct thread *td)
  {
          struct tdq *tdq;
  
          KTR_STATE1(KTR_SCHED, "thread", sched_tdname(td), "runq rem",
              "prio:%d", td->td_priority);
          SDT_PROBE3(sched, , , dequeue, td, td->td_proc, NULL);
          tdq = TDQ_CPU(td_get_sched(td)->ts_cpu);
          TDQ_LOCK_ASSERT(tdq, MA_OWNED);
          MPASS(td->td_lock == TDQ_LOCKPTR(tdq));
          KASSERT(TD_ON_RUNQ(td),
              ("sched_rem: thread not on run queue"));
          tdq_runq_rem(tdq, td);
          tdq_load_rem(tdq, td);
          TD_SET_CAN_RUN(td);
          if (td->td_priority == tdq->tdq_lowpri)
                  tdq_setlowpri(tdq, NULL);
  }
  
  /*
   * Fetch cpu utilization information.  Updates on demand.
   */
  fixpt_t
  sched_pctcpu(struct thread *td)
  {
          fixpt_t pctcpu;
          struct td_sched *ts;
  
          pctcpu = 0;
          ts = td_get_sched(td);
  
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          sched_pctcpu_update(ts, TD_IS_RUNNING(td));
          if (ts->ts_ticks) {
                  int rtick;
  
                  /* How many rtick per second ? */
                  rtick = min(SCHED_TICK_HZ(ts) / SCHED_TICK_SECS, hz);
                  pctcpu = (FSCALE * ((FSCALE * rtick)/hz)) >> FSHIFT;
          }
  
          return (pctcpu);
  }
  
  /*
   * Enforce affinity settings for a thread.  Called after adjustments to
   * cpumask.
   */
  void
  sched_affinity(struct thread *td)
  {
  #ifdef SMP
          struct td_sched *ts;
  
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          ts = td_get_sched(td);
          if (THREAD_CAN_SCHED(td, ts->ts_cpu))
                  return;
          if (TD_ON_RUNQ(td)) {
                  sched_rem(td);
                  sched_add(td, SRQ_BORING | SRQ_HOLDTD);
                  return;
          }
          if (!TD_IS_RUNNING(td))
                  return;
          /*
           * Force a switch before returning to userspace.  If the
           * target thread is not running locally send an ipi to force
           * the issue.
           */
          ast_sched_locked(td, TDA_SCHED);
          if (td != curthread)
                  ipi_cpu(ts->ts_cpu, IPI_PREEMPT);
  #endif
  }
  
  /*
   * Bind a thread to a target cpu.
   */
  void
  sched_bind(struct thread *td, int cpu)
  {
          struct td_sched *ts;
  
          THREAD_LOCK_ASSERT(td, MA_OWNED|MA_NOTRECURSED);
          KASSERT(td == curthread, ("sched_bind: can only bind curthread"));
          ts = td_get_sched(td);
          if (ts->ts_flags & TSF_BOUND)
                  sched_unbind(td);
          KASSERT(THREAD_CAN_MIGRATE(td), ("%p must be migratable", td));
          ts->ts_flags |= TSF_BOUND;
          sched_pin();
          if (PCPU_GET(cpuid) == cpu)
                  return;
          ts->ts_cpu = cpu;
          /* When we return from mi_switch we'll be on the correct cpu. */
          mi_switch(SW_VOL);
          thread_lock(td);
  }
  
  /*
   * Release a bound thread.
   */
  void
  sched_unbind(struct thread *td)
  {
          struct td_sched *ts;
  
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          KASSERT(td == curthread, ("sched_unbind: can only bind curthread"));
          ts = td_get_sched(td);
          if ((ts->ts_flags & TSF_BOUND) == 0)
                  return;
          ts->ts_flags &= ~TSF_BOUND;
          sched_unpin();
  }
  
  int
  sched_is_bound(struct thread *td)
  {
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          return (td_get_sched(td)->ts_flags & TSF_BOUND);
  }
  
  /*
   * Basic yield call.
   */
  void
  sched_relinquish(struct thread *td)
  {
          thread_lock(td);
          mi_switch(SW_VOL | SWT_RELINQUISH);
  }
  
  /*
   * Return the total system load.
   */
  int
  sched_load(void)
  {
  #ifdef SMP
          int total;
          int i;
  
          total = 0;
          CPU_FOREACH(i)
                  total += atomic_load_int(&TDQ_CPU(i)->tdq_sysload);
          return (total);
  #else
          return (atomic_load_int(&TDQ_SELF()->tdq_sysload));
  #endif
  }
  
  int
  sched_sizeof_proc(void)
  {
          return (sizeof(struct proc));
  }
  
  int
  sched_sizeof_thread(void)
  {
          return (sizeof(struct thread) + sizeof(struct td_sched));
  }
  
  #ifdef SMP
  #define TDQ_IDLESPIN(tdq)                                               \
      ((tdq)->tdq_cg != NULL && ((tdq)->tdq_cg->cg_flags & CG_FLAG_THREAD) == 0)
  #else
  #define TDQ_IDLESPIN(tdq)       1
  #endif
  
  /*
   * The actual idle process.
   */
  void
  sched_idletd(void *dummy)
  {
          struct thread *td;
          struct tdq *tdq;
          int oldswitchcnt, switchcnt;
          int i;
  
          mtx_assert(&Giant, MA_NOTOWNED);
          td = curthread;
          tdq = TDQ_SELF();
          THREAD_NO_SLEEPING();
          oldswitchcnt = -1;
          for (;;) {
                  if (TDQ_LOAD(tdq)) {
                          thread_lock(td);
                          mi_switch(SW_VOL | SWT_IDLE);
                  }
                  switchcnt = TDQ_SWITCHCNT(tdq);
  #ifdef SMP
                  if (always_steal || switchcnt != oldswitchcnt) {
                          oldswitchcnt = switchcnt;
                          if (tdq_idled(tdq) == 0)
                                  continue;
                  }
                  switchcnt = TDQ_SWITCHCNT(tdq);
  #else
                  oldswitchcnt = switchcnt;
  #endif
                  /*
                   * If we're switching very frequently, spin while checking
                   * for load rather than entering a low power state that 
                   * may require an IPI.  However, don't do any busy
                   * loops while on SMT machines as this simply steals
                   * cycles from cores doing useful work.
                   */
                  if (TDQ_IDLESPIN(tdq) && switchcnt > sched_idlespinthresh) {
                          for (i = 0; i < sched_idlespins; i++) {
                                  if (TDQ_LOAD(tdq))
                                          break;
                                  cpu_spinwait();
                          }
                  }
  
                  /* If there was context switch during spin, restart it. */
                  switchcnt = TDQ_SWITCHCNT(tdq);
                  if (TDQ_LOAD(tdq) != 0 || switchcnt != oldswitchcnt)
                          continue;
  
                  /* Run main MD idle handler. */
                  atomic_store_int(&tdq->tdq_cpu_idle, 1);
                  /*
                   * Make sure that the tdq_cpu_idle update is globally visible
                   * before cpu_idle() reads tdq_load.  The order is important
                   * to avoid races with tdq_notify().
                   */
                  atomic_thread_fence_seq_cst();
                  /*
                   * Checking for again after the fence picks up assigned
                   * threads often enough to make it worthwhile to do so in
                   * order to avoid calling cpu_idle().
                   */
                  if (TDQ_LOAD(tdq) != 0) {
                          atomic_store_int(&tdq->tdq_cpu_idle, 0);
                          continue;
                  }
                  cpu_idle(switchcnt * 4 > sched_idlespinthresh);
                  atomic_store_int(&tdq->tdq_cpu_idle, 0);
  
                  /*
                   * Account thread-less hardware interrupts and
                   * other wakeup reasons equal to context switches.
                   */
                  switchcnt = TDQ_SWITCHCNT(tdq);
                  if (switchcnt != oldswitchcnt)
                          continue;
                  TDQ_SWITCHCNT_INC(tdq);
                  oldswitchcnt++;
          }
  }
  
  /*
   * sched_throw_grab() chooses a thread from the queue to switch to
   * next.  It returns with the tdq lock dropped in a spinlock section to
   * keep interrupts disabled until the CPU is running in a proper threaded
   * context.
   */
  static struct thread *
  sched_throw_grab(struct tdq *tdq)
  {
          struct thread *newtd;
  
          newtd = choosethread();
          spinlock_enter();
          TDQ_UNLOCK(tdq);
          KASSERT(curthread->td_md.md_spinlock_count == 1,
              ("invalid count %d", curthread->td_md.md_spinlock_count));
          return (newtd);
  }
  
  /*
   * A CPU is entering for the first time.
   */
  void
  sched_ap_entry(void)
  {
          struct thread *newtd;
          struct tdq *tdq;
  
          tdq = TDQ_SELF();
  
          /* This should have been setup in schedinit_ap(). */
          THREAD_LOCKPTR_ASSERT(curthread, TDQ_LOCKPTR(tdq));
  
          TDQ_LOCK(tdq);
          /* Correct spinlock nesting. */
          spinlock_exit();
          PCPU_SET(switchtime, cpu_ticks());
          PCPU_SET(switchticks, ticks);
  
          newtd = sched_throw_grab(tdq);
  
          /* doesn't return */
          cpu_throw(NULL, newtd);
  }
  
  /*
   * A thread is exiting.
   */
  void
  sched_throw(struct thread *td)
  {
          struct thread *newtd;
          struct tdq *tdq;
  
          tdq = TDQ_SELF();
  
          MPASS(td != NULL);
          THREAD_LOCK_ASSERT(td, MA_OWNED);
          THREAD_LOCKPTR_ASSERT(td, TDQ_LOCKPTR(tdq));
  
          tdq_load_rem(tdq, td);
          td->td_lastcpu = td->td_oncpu;
          td->td_oncpu = NOCPU;
          thread_lock_block(td);
  
          newtd = sched_throw_grab(tdq);
  
          /* doesn't return */
          cpu_switch(td, newtd, TDQ_LOCKPTR(tdq));
  }
  
  /*
   * This is called from fork_exit().  Just acquire the correct locks and
   * let fork do the rest of the work.
   */
  void
  sched_fork_exit(struct thread *td)
  {
          struct tdq *tdq;
          int cpuid;
  
          /*
           * Finish setting up thread glue so that it begins execution in a
           * non-nested critical section with the scheduler lock held.
           */
          KASSERT(curthread->td_md.md_spinlock_count == 1,
              ("invalid count %d", curthread->td_md.md_spinlock_count));
          cpuid = PCPU_GET(cpuid);
          tdq = TDQ_SELF();
          TDQ_LOCK(tdq);
          spinlock_exit();
          MPASS(td->td_lock == TDQ_LOCKPTR(tdq));
          td->td_oncpu = cpuid;
          KTR_STATE1(KTR_SCHED, "thread", sched_tdname(td), "running",
              "prio:%d", td->td_priority);
          SDT_PROBE0(sched, , , on__cpu);
  }
  
  /*
   * Create on first use to catch odd startup conditions.
   */
  char *
  sched_tdname(struct thread *td)
  {
  #ifdef KTR
          struct td_sched *ts;
  
          ts = td_get_sched(td);
          if (ts->ts_name[0] == '\0')
                  snprintf(ts->ts_name, sizeof(ts->ts_name),
                      "%s tid %d", td->td_name, td->td_tid);
          return (ts->ts_name);
  #else
          return (td->td_name);
  #endif
  }
  
  #ifdef KTR
  void
  sched_clear_tdname(struct thread *td)
  {
          struct td_sched *ts;
  
          ts = td_get_sched(td);
          ts->ts_name[0] = '\0';
  }
  #endif
  
  #ifdef SMP
  
  /*
   * Build the CPU topology dump string. Is recursively called to collect
   * the topology tree.
   */
  static int
  sysctl_kern_sched_topology_spec_internal(struct sbuf *sb, struct cpu_group *cg,
      int indent)
  {
          char cpusetbuf[CPUSETBUFSIZ];
          int i, first;
  
          sbuf_printf(sb, "%*s<group level=\"%d\" cache-level=\"%d\">\n", indent,
              "", 1 + indent / 2, cg->cg_level);
          sbuf_printf(sb, "%*s <cpu count=\"%d\" mask=\"%s\">", indent, "",
              cg->cg_count, cpusetobj_strprint(cpusetbuf, &cg->cg_mask));
          first = TRUE;
          for (i = cg->cg_first; i <= cg->cg_last; i++) {
                  if (CPU_ISSET(i, &cg->cg_mask)) {
                          if (!first)
                                  sbuf_printf(sb, ", ");
                          else
                                  first = FALSE;
                          sbuf_printf(sb, "%d", i);
                  }
          }
          sbuf_printf(sb, "</cpu>\n");
  
          if (cg->cg_flags != 0) {
                  sbuf_printf(sb, "%*s <flags>", indent, "");
                  if ((cg->cg_flags & CG_FLAG_HTT) != 0)
                          sbuf_printf(sb, "<flag name=\"HTT\">HTT group</flag>");
                  if ((cg->cg_flags & CG_FLAG_THREAD) != 0)
                          sbuf_printf(sb, "<flag name=\"THREAD\">THREAD group</flag>");
                  if ((cg->cg_flags & CG_FLAG_SMT) != 0)
                          sbuf_printf(sb, "<flag name=\"SMT\">SMT group</flag>");
                  if ((cg->cg_flags & CG_FLAG_NODE) != 0)
                          sbuf_printf(sb, "<flag name=\"NODE\">NUMA node</flag>");
                  sbuf_printf(sb, "</flags>\n");
          }
  
          if (cg->cg_children > 0) {
                  sbuf_printf(sb, "%*s <children>\n", indent, "");
                  for (i = 0; i < cg->cg_children; i++)
                          sysctl_kern_sched_topology_spec_internal(sb, 
                              &cg->cg_child[i], indent+2);
                  sbuf_printf(sb, "%*s </children>\n", indent, "");
          }
          sbuf_printf(sb, "%*s</group>\n", indent, "");
          return (0);
  }
  
  /*
   * Sysctl handler for retrieving topology dump. It's a wrapper for
   * the recursive sysctl_kern_smp_topology_spec_internal().
   */
  static int
  sysctl_kern_sched_topology_spec(SYSCTL_HANDLER_ARGS)
  {
          struct sbuf *topo;
          int err;
  
          KASSERT(cpu_top != NULL, ("cpu_top isn't initialized"));
  
          topo = sbuf_new_for_sysctl(NULL, NULL, 512, req);
          if (topo == NULL)
                  return (ENOMEM);
  
          sbuf_printf(topo, "<groups>\n");
          err = sysctl_kern_sched_topology_spec_internal(topo, cpu_top, 1);
          sbuf_printf(topo, "</groups>\n");
  
          if (err == 0) {
                  err = sbuf_finish(topo);
          }
          sbuf_delete(topo);
          return (err);
  }
  
  #endif
  
  static int
  sysctl_kern_quantum(SYSCTL_HANDLER_ARGS)
  {
          int error, new_val, period;
  
          period = 1000000 / realstathz;
          new_val = period * sched_slice;
          error = sysctl_handle_int(oidp, &new_val, 0, req);
          if (error != 0 || req->newptr == NULL)
                  return (error);
          if (new_val <= 0)
                  return (EINVAL);
          sched_slice = imax(1, (new_val + period / 2) / period);
          sched_slice_min = sched_slice / SCHED_SLICE_MIN_DIVISOR;
          hogticks = imax(1, (2 * hz * sched_slice + realstathz / 2) /
              realstathz);
          return (0);
  }
  
  SYSCTL_NODE(_kern, OID_AUTO, sched, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
      "Scheduler");
  SYSCTL_STRING(_kern_sched, OID_AUTO, name, CTLFLAG_RD, "ULE", 0,
      "Scheduler name");
  SYSCTL_PROC(_kern_sched, OID_AUTO, quantum,
      CTLTYPE_INT | CTLFLAG_RW | CTLFLAG_MPSAFE, NULL, 0,
      sysctl_kern_quantum, "I",
      "Quantum for timeshare threads in microseconds");
  SYSCTL_INT(_kern_sched, OID_AUTO, slice, CTLFLAG_RW, &sched_slice, 0,
      "Quantum for timeshare threads in stathz ticks");
  SYSCTL_UINT(_kern_sched, OID_AUTO, interact, CTLFLAG_RW, &sched_interact, 0,
      "Interactivity score threshold");
  SYSCTL_INT(_kern_sched, OID_AUTO, preempt_thresh, CTLFLAG_RW,
      &preempt_thresh, 0,
      "Maximal (lowest) priority for preemption");
  SYSCTL_INT(_kern_sched, OID_AUTO, static_boost, CTLFLAG_RW, &static_boost, 0,
      "Assign static kernel priorities to sleeping threads");
  SYSCTL_INT(_kern_sched, OID_AUTO, idlespins, CTLFLAG_RW, &sched_idlespins, 0,
      "Number of times idle thread will spin waiting for new work");
  SYSCTL_INT(_kern_sched, OID_AUTO, idlespinthresh, CTLFLAG_RW,
      &sched_idlespinthresh, 0,
      "Threshold before we will permit idle thread spinning");
  #ifdef SMP
  SYSCTL_INT(_kern_sched, OID_AUTO, affinity, CTLFLAG_RW, &affinity, 0,
      "Number of hz ticks to keep thread affinity for");
  SYSCTL_INT(_kern_sched, OID_AUTO, balance, CTLFLAG_RW, &rebalance, 0,
      "Enables the long-term load balancer");
  SYSCTL_INT(_kern_sched, OID_AUTO, balance_interval, CTLFLAG_RW,
      &balance_interval, 0,
      "Average period in stathz ticks to run the long-term balancer");
  SYSCTL_INT(_kern_sched, OID_AUTO, steal_idle, CTLFLAG_RW, &steal_idle, 0,
      "Attempts to steal work from other cores before idling");
  SYSCTL_INT(_kern_sched, OID_AUTO, steal_thresh, CTLFLAG_RW, &steal_thresh, 0,
      "Minimum load on remote CPU before we'll steal");
  SYSCTL_INT(_kern_sched, OID_AUTO, trysteal_limit, CTLFLAG_RW, &trysteal_limit,
      0, "Topological distance limit for stealing threads in sched_switch()");
  SYSCTL_INT(_kern_sched, OID_AUTO, always_steal, CTLFLAG_RW, &always_steal, 0,
      "Always run the stealer from the idle thread");
  SYSCTL_PROC(_kern_sched, OID_AUTO, topology_spec, CTLTYPE_STRING |
      CTLFLAG_MPSAFE | CTLFLAG_RD, NULL, 0, sysctl_kern_sched_topology_spec, "A",
      "XML dump of detected CPU topology");
  #endif
  
  /* ps compat.  All cpu percentages from ULE are weighted. */
  static int ccpu = 0;
  SYSCTL_INT(_kern, OID_AUTO, ccpu, CTLFLAG_RD, &ccpu, 0,
      "Decay factor used for updating %CPU in 4BSD scheduler");