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

     /*-
      * Copyright (c) 1982, 1986, 1991, 1993
      *      The Regents of the University of California.  All rights reserved.
      * (c) UNIX System Laboratories, Inc.
      * All or some portions of this file are derived from material licensed
      * to the University of California by American Telephone and Telegraph
      * Co. or Unix System Laboratories, Inc. and are reproduced herein with
      * the permission of UNIX System Laboratories, Inc.
      *
     * 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, 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.
     * 4. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``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 REGENTS OR CONTRIBUTORS 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.
     *
     *      From: @(#)kern_clock.c  8.5 (Berkeley) 1/21/94
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include "opt_kdtrace.h"
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/bus.h>
    #include <sys/callout.h>
    #include <sys/condvar.h>
    #include <sys/interrupt.h>
    #include <sys/kernel.h>
    #include <sys/ktr.h>
    #include <sys/lock.h>
    #include <sys/malloc.h>
    #include <sys/mutex.h>
    #include <sys/proc.h>
    #include <sys/sdt.h>
    #include <sys/sleepqueue.h>
    #include <sys/sysctl.h>
    #include <sys/smp.h>
    
    #ifdef SMP
    #include <machine/cpu.h>
    #endif
    
    SDT_PROVIDER_DEFINE(callout_execute);
    SDT_PROBE_DEFINE1(callout_execute, kernel, , callout__start,
        "struct callout *");
    SDT_PROBE_DEFINE1(callout_execute, kernel, , callout__end,
        "struct callout *");
    
    static int avg_depth;
    SYSCTL_INT(_debug, OID_AUTO, to_avg_depth, CTLFLAG_RD, &avg_depth, 0,
        "Average number of items examined per softclock call. Units = 1/1000");
    static int avg_gcalls;
    SYSCTL_INT(_debug, OID_AUTO, to_avg_gcalls, CTLFLAG_RD, &avg_gcalls, 0,
        "Average number of Giant callouts made per softclock call. Units = 1/1000");
    static int avg_lockcalls;
    SYSCTL_INT(_debug, OID_AUTO, to_avg_lockcalls, CTLFLAG_RD, &avg_lockcalls, 0,
        "Average number of lock callouts made per softclock call. Units = 1/1000");
    static int avg_mpcalls;
    SYSCTL_INT(_debug, OID_AUTO, to_avg_mpcalls, CTLFLAG_RD, &avg_mpcalls, 0,
        "Average number of MP callouts made per softclock call. Units = 1/1000");
    /*
     * TODO:
     *      allocate more timeout table slots when table overflows.
     */
    int callwheelsize, callwheelbits, callwheelmask;
    
    /*
     * The callout cpu migration entity represents informations necessary for
     * describing the migrating callout to the new callout cpu.
     * The cached informations are very important for deferring migration when
     * the migrating callout is already running.
     */
    struct cc_mig_ent {
    #ifdef SMP
            void    (*ce_migration_func)(void *);
            void    *ce_migration_arg;
            int     ce_migration_cpu;
            int     ce_migration_ticks;
    #endif
   };
           
   /*
    * There is one struct callout_cpu per cpu, holding all relevant
    * state for the callout processing thread on the individual CPU.
    * In particular:
    *      cc_ticks is incremented once per tick in callout_cpu().
    *      It tracks the global 'ticks' but in a way that the individual
    *      threads should not worry about races in the order in which
    *      hardclock() and hardclock_cpu() run on the various CPUs.
    *      cc_softclock is advanced in callout_cpu() to point to the
    *      first entry in cc_callwheel that may need handling. In turn,
    *      a softclock() is scheduled so it can serve the various entries i
    *      such that cc_softclock <= i <= cc_ticks .
    *      XXX maybe cc_softclock and cc_ticks should be volatile ?
    *
    *      cc_ticks is also used in callout_reset_cpu() to determine
    *      when the callout should be served.
    */
   struct callout_cpu {
           struct cc_mig_ent       cc_migrating_entity;
           struct mtx              cc_lock;
           struct callout          *cc_callout;
           struct callout_tailq    *cc_callwheel;
           struct callout_list     cc_callfree;
           struct callout          *cc_next;
           struct callout          *cc_curr;
           void                    *cc_cookie;
           int                     cc_ticks;
           int                     cc_softticks;
           int                     cc_cancel;
           int                     cc_waiting;
           int                     cc_firsttick;
   };
   
   #ifdef SMP
   #define cc_migration_func       cc_migrating_entity.ce_migration_func
   #define cc_migration_arg        cc_migrating_entity.ce_migration_arg
   #define cc_migration_cpu        cc_migrating_entity.ce_migration_cpu
   #define cc_migration_ticks      cc_migrating_entity.ce_migration_ticks
   
   struct callout_cpu cc_cpu[MAXCPU];
   #define CPUBLOCK        MAXCPU
   #define CC_CPU(cpu)     (&cc_cpu[(cpu)])
   #define CC_SELF()       CC_CPU(PCPU_GET(cpuid))
   #else
   struct callout_cpu cc_cpu;
   #define CC_CPU(cpu)     &cc_cpu
   #define CC_SELF()       &cc_cpu
   #endif
   #define CC_LOCK(cc)     mtx_lock_spin(&(cc)->cc_lock)
   #define CC_UNLOCK(cc)   mtx_unlock_spin(&(cc)->cc_lock)
   #define CC_LOCK_ASSERT(cc)      mtx_assert(&(cc)->cc_lock, MA_OWNED)
   
   static int timeout_cpu;
   void (*callout_new_inserted)(int cpu, int ticks) = NULL;
   
   static MALLOC_DEFINE(M_CALLOUT, "callout", "Callout datastructures");
   
   /**
    * Locked by cc_lock:
    *   cc_curr         - If a callout is in progress, it is curr_callout.
    *                     If curr_callout is non-NULL, threads waiting in
    *                     callout_drain() will be woken up as soon as the
    *                     relevant callout completes.
    *   cc_cancel       - Changing to 1 with both callout_lock and c_lock held
    *                     guarantees that the current callout will not run.
    *                     The softclock() function sets this to 0 before it
    *                     drops callout_lock to acquire c_lock, and it calls
    *                     the handler only if curr_cancelled is still 0 after
    *                     c_lock is successfully acquired.
    *   cc_waiting      - If a thread is waiting in callout_drain(), then
    *                     callout_wait is nonzero.  Set only when
    *                     curr_callout is non-NULL.
    */
   
   /*
    * Resets the migration entity tied to a specific callout cpu.
    */
   static void
   cc_cme_cleanup(struct callout_cpu *cc)
   {
   
   #ifdef SMP
           cc->cc_migration_cpu = CPUBLOCK;
           cc->cc_migration_ticks = 0;
           cc->cc_migration_func = NULL;
           cc->cc_migration_arg = NULL;
   #endif
   }
   
   /*
    * Checks if migration is requested by a specific callout cpu.
    */
   static int
   cc_cme_migrating(struct callout_cpu *cc)
   {
   
   #ifdef SMP
           return (cc->cc_migration_cpu != CPUBLOCK);
   #else
           return (0);
   #endif
   }
   
   /*
    * kern_timeout_callwheel_alloc() - kernel low level callwheel initialization 
    *
    *      This code is called very early in the kernel initialization sequence,
    *      and may be called more then once.
    */
   caddr_t
   kern_timeout_callwheel_alloc(caddr_t v)
   {
           struct callout_cpu *cc;
   
           timeout_cpu = PCPU_GET(cpuid);
           cc = CC_CPU(timeout_cpu);
           /*
            * Calculate callout wheel size
            */
           for (callwheelsize = 1, callwheelbits = 0;
                callwheelsize < ncallout;
                callwheelsize <<= 1, ++callwheelbits)
                   ;
           callwheelmask = callwheelsize - 1;
   
           cc->cc_callout = (struct callout *)v;
           v = (caddr_t)(cc->cc_callout + ncallout);
           cc->cc_callwheel = (struct callout_tailq *)v;
           v = (caddr_t)(cc->cc_callwheel + callwheelsize);
           return(v);
   }
   
   static void
   callout_cpu_init(struct callout_cpu *cc)
   {
           struct callout *c;
           int i;
   
           mtx_init(&cc->cc_lock, "callout", NULL, MTX_SPIN | MTX_RECURSE);
           SLIST_INIT(&cc->cc_callfree);
           for (i = 0; i < callwheelsize; i++) {
                   TAILQ_INIT(&cc->cc_callwheel[i]);
           }
           cc_cme_cleanup(cc);
           if (cc->cc_callout == NULL)
                   return;
           for (i = 0; i < ncallout; i++) {
                   c = &cc->cc_callout[i];
                   callout_init(c, 0);
                   c->c_flags = CALLOUT_LOCAL_ALLOC;
                   SLIST_INSERT_HEAD(&cc->cc_callfree, c, c_links.sle);
           }
   }
   
   #ifdef SMP
   /*
    * Switches the cpu tied to a specific callout.
    * The function expects a locked incoming callout cpu and returns with
    * locked outcoming callout cpu.
    */
   static struct callout_cpu *
   callout_cpu_switch(struct callout *c, struct callout_cpu *cc, int new_cpu)
   {
           struct callout_cpu *new_cc;
   
           MPASS(c != NULL && cc != NULL);
           CC_LOCK_ASSERT(cc);
   
           /*
            * Avoid interrupts and preemption firing after the callout cpu
            * is blocked in order to avoid deadlocks as the new thread
            * may be willing to acquire the callout cpu lock.
            */
           c->c_cpu = CPUBLOCK;
           spinlock_enter();
           CC_UNLOCK(cc);
           new_cc = CC_CPU(new_cpu);
           CC_LOCK(new_cc);
           spinlock_exit();
           c->c_cpu = new_cpu;
           return (new_cc);
   }
   #endif
   
   /*
    * kern_timeout_callwheel_init() - initialize previously reserved callwheel
    *                                 space.
    *
    *      This code is called just once, after the space reserved for the
    *      callout wheel has been finalized.
    */
   void
   kern_timeout_callwheel_init(void)
   {
           callout_cpu_init(CC_CPU(timeout_cpu));
   }
   
   /*
    * Start standard softclock thread.
    */
   static void
   start_softclock(void *dummy)
   {
           struct callout_cpu *cc;
   #ifdef SMP
           int cpu;
   #endif
   
           cc = CC_CPU(timeout_cpu);
           if (swi_add(&clk_intr_event, "clock", softclock, cc, SWI_CLOCK,
               INTR_MPSAFE, &cc->cc_cookie))
                   panic("died while creating standard software ithreads");
   #ifdef SMP
           CPU_FOREACH(cpu) {
                   if (cpu == timeout_cpu)
                           continue;
                   cc = CC_CPU(cpu);
                   if (swi_add(NULL, "clock", softclock, cc, SWI_CLOCK,
                       INTR_MPSAFE, &cc->cc_cookie))
                           panic("died while creating standard software ithreads");
                   cc->cc_callout = NULL;  /* Only cpu0 handles timeout(). */
                   cc->cc_callwheel = malloc(
                       sizeof(struct callout_tailq) * callwheelsize, M_CALLOUT,
                       M_WAITOK);
                   callout_cpu_init(cc);
           }
   #endif
   }
   
   SYSINIT(start_softclock, SI_SUB_SOFTINTR, SI_ORDER_FIRST, start_softclock, NULL);
   
   void
   callout_tick(void)
   {
           struct callout_cpu *cc;
           int need_softclock;
           int bucket;
   
           /*
            * Process callouts at a very low cpu priority, so we don't keep the
            * relatively high clock interrupt priority any longer than necessary.
            */
           need_softclock = 0;
           cc = CC_SELF();
           mtx_lock_spin_flags(&cc->cc_lock, MTX_QUIET);
           cc->cc_firsttick = cc->cc_ticks = ticks;
           for (; (cc->cc_softticks - cc->cc_ticks) <= 0; cc->cc_softticks++) {
                   bucket = cc->cc_softticks & callwheelmask;
                   if (!TAILQ_EMPTY(&cc->cc_callwheel[bucket])) {
                           need_softclock = 1;
                           break;
                   }
           }
           mtx_unlock_spin_flags(&cc->cc_lock, MTX_QUIET);
           /*
            * swi_sched acquires the thread lock, so we don't want to call it
            * with cc_lock held; incorrect locking order.
            */
           if (need_softclock)
                   swi_sched(cc->cc_cookie, 0);
   }
   
   int
   callout_tickstofirst(int limit)
   {
           struct callout_cpu *cc;
           struct callout *c;
           struct callout_tailq *sc;
           int curticks;
           int skip = 1;
   
           cc = CC_SELF();
           mtx_lock_spin_flags(&cc->cc_lock, MTX_QUIET);
           curticks = cc->cc_ticks;
           while( skip < ncallout && skip < limit ) {
                   sc = &cc->cc_callwheel[ (curticks+skip) & callwheelmask ];
                   /* search scanning ticks */
                   TAILQ_FOREACH( c, sc, c_links.tqe ){
                           if (c->c_time - curticks <= ncallout)
                                   goto out;
                   }
                   skip++;
           }
   out:
           cc->cc_firsttick = curticks + skip;
           mtx_unlock_spin_flags(&cc->cc_lock, MTX_QUIET);
           return (skip);
   }
   
   static struct callout_cpu *
   callout_lock(struct callout *c)
   {
           struct callout_cpu *cc;
           int cpu;
   
           for (;;) {
                   cpu = c->c_cpu;
   #ifdef SMP
                   if (cpu == CPUBLOCK) {
                           while (c->c_cpu == CPUBLOCK)
                                   cpu_spinwait();
                           continue;
                   }
   #endif
                   cc = CC_CPU(cpu);
                   CC_LOCK(cc);
                   if (cpu == c->c_cpu)
                           break;
                   CC_UNLOCK(cc);
           }
           return (cc);
   }
   
   static void
   callout_cc_add(struct callout *c, struct callout_cpu *cc, int to_ticks,
       void (*func)(void *), void *arg, int cpu)
   {
   
           CC_LOCK_ASSERT(cc);
   
           if (to_ticks <= 0)
                   to_ticks = 1;
           c->c_arg = arg;
           c->c_flags |= (CALLOUT_ACTIVE | CALLOUT_PENDING);
           c->c_func = func;
           c->c_time = ticks + to_ticks;
           TAILQ_INSERT_TAIL(&cc->cc_callwheel[c->c_time & callwheelmask], 
               c, c_links.tqe);
           if ((c->c_time - cc->cc_firsttick) < 0 &&
               callout_new_inserted != NULL) {
                   cc->cc_firsttick = c->c_time;
                   (*callout_new_inserted)(cpu,
                       to_ticks + (ticks - cc->cc_ticks));
           }
   }
   
   static void
   callout_cc_del(struct callout *c, struct callout_cpu *cc)
   {
   
           if ((c->c_flags & CALLOUT_LOCAL_ALLOC) == 0)
                   return;
           c->c_func = NULL;
           SLIST_INSERT_HEAD(&cc->cc_callfree, c, c_links.sle);
   }
   
   static void
   softclock_call_cc(struct callout *c, struct callout_cpu *cc, int *mpcalls,
       int *lockcalls, int *gcalls)
   {
           void (*c_func)(void *);
           void *c_arg;
           struct lock_class *class;
           struct lock_object *c_lock;
           int c_flags, sharedlock;
   #ifdef SMP
           struct callout_cpu *new_cc;
           void (*new_func)(void *);
           void *new_arg;
           int new_cpu, new_ticks;
   #endif
   #ifdef DIAGNOSTIC
           struct bintime bt1, bt2;
           struct timespec ts2;
           static uint64_t maxdt = 36893488147419102LL;    /* 2 msec */
           static timeout_t *lastfunc;
   #endif
   
           KASSERT((c->c_flags & (CALLOUT_PENDING | CALLOUT_ACTIVE)) ==
               (CALLOUT_PENDING | CALLOUT_ACTIVE),
               ("softclock_call_cc: pend|act %p %x", c, c->c_flags));
           class = (c->c_lock != NULL) ? LOCK_CLASS(c->c_lock) : NULL;
           sharedlock = (c->c_flags & CALLOUT_SHAREDLOCK) ? 0 : 1;
           c_lock = c->c_lock;
           c_func = c->c_func;
           c_arg = c->c_arg;
           c_flags = c->c_flags;
           if (c->c_flags & CALLOUT_LOCAL_ALLOC)
                   c->c_flags = CALLOUT_LOCAL_ALLOC;
           else
                   c->c_flags &= ~CALLOUT_PENDING;
           cc->cc_curr = c;
           cc->cc_cancel = 0;
           CC_UNLOCK(cc);
           if (c_lock != NULL) {
                   class->lc_lock(c_lock, sharedlock);
                   /*
                    * The callout may have been cancelled
                    * while we switched locks.
                    */
                   if (cc->cc_cancel) {
                           class->lc_unlock(c_lock);
                           goto skip;
                   }
                   /* The callout cannot be stopped now. */
                   cc->cc_cancel = 1;
   
                   if (c_lock == &Giant.lock_object) {
                           (*gcalls)++;
                           CTR3(KTR_CALLOUT, "callout %p func %p arg %p",
                               c, c_func, c_arg);
                   } else {
                           (*lockcalls)++;
                           CTR3(KTR_CALLOUT, "callout lock %p func %p arg %p",
                               c, c_func, c_arg);
                   }
           } else {
                   (*mpcalls)++;
                   CTR3(KTR_CALLOUT, "callout mpsafe %p func %p arg %p",
                       c, c_func, c_arg);
           }
   #ifdef DIAGNOSTIC
           binuptime(&bt1);
   #endif
           THREAD_NO_SLEEPING();
           SDT_PROBE1(callout_execute, kernel, , callout__start, c);
           c_func(c_arg);
           SDT_PROBE1(callout_execute, kernel, , callout__end, c);
           THREAD_SLEEPING_OK();
   #ifdef DIAGNOSTIC
           binuptime(&bt2);
           bintime_sub(&bt2, &bt1);
           if (bt2.frac > maxdt) {
                   if (lastfunc != c_func || bt2.frac > maxdt * 2) {
                           bintime2timespec(&bt2, &ts2);
                           printf(
                   "Expensive timeout(9) function: %p(%p) %jd.%09ld s\n",
                               c_func, c_arg, (intmax_t)ts2.tv_sec, ts2.tv_nsec);
                   }
                   maxdt = bt2.frac;
                   lastfunc = c_func;
           }
   #endif
           CTR1(KTR_CALLOUT, "callout %p finished", c);
           if ((c_flags & CALLOUT_RETURNUNLOCKED) == 0)
                   class->lc_unlock(c_lock);
   skip:
           CC_LOCK(cc);
           KASSERT(cc->cc_curr == c, ("mishandled cc_curr"));
           cc->cc_curr = NULL;
           if (cc->cc_waiting) {
                   /*
                    * There is someone waiting for the
                    * callout to complete.
                    * If the callout was scheduled for
                    * migration just cancel it.
                    */
                   if (cc_cme_migrating(cc)) {
                           cc_cme_cleanup(cc);
   
                           /*
                            * It should be assert here that the callout is not
                            * destroyed but that is not easy.
                            */
                           c->c_flags &= ~CALLOUT_DFRMIGRATION;
                   }
                   cc->cc_waiting = 0;
                   CC_UNLOCK(cc);
                   wakeup(&cc->cc_waiting);
                   CC_LOCK(cc);
           } else if (cc_cme_migrating(cc)) {
                   KASSERT((c_flags & CALLOUT_LOCAL_ALLOC) == 0,
                       ("Migrating legacy callout %p", c));
   #ifdef SMP
                   /*
                    * If the callout was scheduled for
                    * migration just perform it now.
                    */
                   new_cpu = cc->cc_migration_cpu;
                   new_ticks = cc->cc_migration_ticks;
                   new_func = cc->cc_migration_func;
                   new_arg = cc->cc_migration_arg;
                   cc_cme_cleanup(cc);
   
                   /*
                    * It should be assert here that the callout is not destroyed
                    * but that is not easy.
                    *
                    * As first thing, handle deferred callout stops.
                    */
                   if ((c->c_flags & CALLOUT_DFRMIGRATION) == 0) {
                           CTR3(KTR_CALLOUT,
                                "deferred cancelled %p func %p arg %p",
                                c, new_func, new_arg);
                           callout_cc_del(c, cc);
                           return;
                   }
                   c->c_flags &= ~CALLOUT_DFRMIGRATION;
   
                   new_cc = callout_cpu_switch(c, cc, new_cpu);
                   callout_cc_add(c, new_cc, new_ticks, new_func, new_arg,
                       new_cpu);
                   CC_UNLOCK(new_cc);
                   CC_LOCK(cc);
   #else
                   panic("migration should not happen");
   #endif
           }
           /*
            * If the current callout is locally allocated (from
            * timeout(9)) then put it on the freelist.
            *
            * Note: we need to check the cached copy of c_flags because
            * if it was not local, then it's not safe to deref the
            * callout pointer.
            */
           KASSERT((c_flags & CALLOUT_LOCAL_ALLOC) == 0 ||
               c->c_flags == CALLOUT_LOCAL_ALLOC,
               ("corrupted callout"));
           if (c_flags & CALLOUT_LOCAL_ALLOC)
                   callout_cc_del(c, cc);
   }
   
   /*
    * The callout mechanism is based on the work of Adam M. Costello and 
    * George Varghese, published in a technical report entitled "Redesigning
    * the BSD Callout and Timer Facilities" and modified slightly for inclusion
    * in FreeBSD by Justin T. Gibbs.  The original work on the data structures
    * used in this implementation was published by G. Varghese and T. Lauck in
    * the paper "Hashed and Hierarchical Timing Wheels: Data Structures for
    * the Efficient Implementation of a Timer Facility" in the Proceedings of
    * the 11th ACM Annual Symposium on Operating Systems Principles,
    * Austin, Texas Nov 1987.
    */
   
   /*
    * Software (low priority) clock interrupt.
    * Run periodic events from timeout queue.
    */
   void
   softclock(void *arg)
   {
           struct callout_cpu *cc;
           struct callout *c;
           struct callout_tailq *bucket;
           int curticks;
           int steps;      /* #steps since we last allowed interrupts */
           int depth;
           int mpcalls;
           int lockcalls;
           int gcalls;
   
   #ifndef MAX_SOFTCLOCK_STEPS
   #define MAX_SOFTCLOCK_STEPS 100 /* Maximum allowed value of steps. */
   #endif /* MAX_SOFTCLOCK_STEPS */
   
           mpcalls = 0;
           lockcalls = 0;
           gcalls = 0;
           depth = 0;
           steps = 0;
           cc = (struct callout_cpu *)arg;
           CC_LOCK(cc);
           while (cc->cc_softticks - 1 != cc->cc_ticks) {
                   /*
                    * cc_softticks may be modified by hard clock, so cache
                    * it while we work on a given bucket.
                    */
                   curticks = cc->cc_softticks;
                   cc->cc_softticks++;
                   bucket = &cc->cc_callwheel[curticks & callwheelmask];
                   c = TAILQ_FIRST(bucket);
                   while (c != NULL) {
                           depth++;
                           if (c->c_time != curticks) {
                                   c = TAILQ_NEXT(c, c_links.tqe);
                                   ++steps;
                                   if (steps >= MAX_SOFTCLOCK_STEPS) {
                                           cc->cc_next = c;
                                           /* Give interrupts a chance. */
                                           CC_UNLOCK(cc);
                                           ;       /* nothing */
                                           CC_LOCK(cc);
                                           c = cc->cc_next;
                                           steps = 0;
                                   }
                           } else {
                                   cc->cc_next = TAILQ_NEXT(c, c_links.tqe);
                                   TAILQ_REMOVE(bucket, c, c_links.tqe);
                                   softclock_call_cc(c, cc, &mpcalls,
                                       &lockcalls, &gcalls);
                                   steps = 0;
                                   c = cc->cc_next;
                           }
                   }
           }
           avg_depth += (depth * 1000 - avg_depth) >> 8;
           avg_mpcalls += (mpcalls * 1000 - avg_mpcalls) >> 8;
           avg_lockcalls += (lockcalls * 1000 - avg_lockcalls) >> 8;
           avg_gcalls += (gcalls * 1000 - avg_gcalls) >> 8;
           cc->cc_next = NULL;
           CC_UNLOCK(cc);
   }
   
   /*
    * timeout --
    *      Execute a function after a specified length of time.
    *
    * untimeout --
    *      Cancel previous timeout function call.
    *
    * callout_handle_init --
    *      Initialize a handle so that using it with untimeout is benign.
    *
    *      See AT&T BCI Driver Reference Manual for specification.  This
    *      implementation differs from that one in that although an 
    *      identification value is returned from timeout, the original
    *      arguments to timeout as well as the identifier are used to
    *      identify entries for untimeout.
    */
   struct callout_handle
   timeout(ftn, arg, to_ticks)
           timeout_t *ftn;
           void *arg;
           int to_ticks;
   {
           struct callout_cpu *cc;
           struct callout *new;
           struct callout_handle handle;
   
           cc = CC_CPU(timeout_cpu);
           CC_LOCK(cc);
           /* Fill in the next free callout structure. */
           new = SLIST_FIRST(&cc->cc_callfree);
           if (new == NULL)
                   /* XXX Attempt to malloc first */
                   panic("timeout table full");
           SLIST_REMOVE_HEAD(&cc->cc_callfree, c_links.sle);
           callout_reset(new, to_ticks, ftn, arg);
           handle.callout = new;
           CC_UNLOCK(cc);
   
           return (handle);
   }
   
   void
   untimeout(ftn, arg, handle)
           timeout_t *ftn;
           void *arg;
           struct callout_handle handle;
   {
           struct callout_cpu *cc;
   
           /*
            * Check for a handle that was initialized
            * by callout_handle_init, but never used
            * for a real timeout.
            */
           if (handle.callout == NULL)
                   return;
   
           cc = callout_lock(handle.callout);
           if (handle.callout->c_func == ftn && handle.callout->c_arg == arg)
                   callout_stop(handle.callout);
           CC_UNLOCK(cc);
   }
   
   void
   callout_handle_init(struct callout_handle *handle)
   {
           handle->callout = NULL;
   }
   
   /*
    * New interface; clients allocate their own callout structures.
    *
    * callout_reset() - establish or change a timeout
    * callout_stop() - disestablish a timeout
    * callout_init() - initialize a callout structure so that it can
    *      safely be passed to callout_reset() and callout_stop()
    *
    * <sys/callout.h> defines three convenience macros:
    *
    * callout_active() - returns truth if callout has not been stopped,
    *      drained, or deactivated since the last time the callout was
    *      reset.
    * callout_pending() - returns truth if callout is still waiting for timeout
    * callout_deactivate() - marks the callout as having been serviced
    */
   int
   callout_reset_on(struct callout *c, int to_ticks, void (*ftn)(void *),
       void *arg, int cpu)
   {
           struct callout_cpu *cc;
           int cancelled = 0;
   
           /*
            * Don't allow migration of pre-allocated callouts lest they
            * become unbalanced.
            */
           if (c->c_flags & CALLOUT_LOCAL_ALLOC)
                   cpu = c->c_cpu;
           cc = callout_lock(c);
           if (cc->cc_curr == c) {
                   /*
                    * We're being asked to reschedule a callout which is
                    * currently in progress.  If there is a lock then we
                    * can cancel the callout if it has not really started.
                    */
                   if (c->c_lock != NULL && !cc->cc_cancel)
                           cancelled = cc->cc_cancel = 1;
                   if (cc->cc_waiting) {
                           /*
                            * Someone has called callout_drain to kill this
                            * callout.  Don't reschedule.
                            */
                           CTR4(KTR_CALLOUT, "%s %p func %p arg %p",
                               cancelled ? "cancelled" : "failed to cancel",
                               c, c->c_func, c->c_arg);
                           CC_UNLOCK(cc);
                           return (cancelled);
                   }
           }
           if (c->c_flags & CALLOUT_PENDING) {
                   if (cc->cc_next == c) {
                           cc->cc_next = TAILQ_NEXT(c, c_links.tqe);
                   }
                   TAILQ_REMOVE(&cc->cc_callwheel[c->c_time & callwheelmask], c,
                       c_links.tqe);
   
                   cancelled = 1;
                   c->c_flags &= ~(CALLOUT_ACTIVE | CALLOUT_PENDING);
           }
   
   #ifdef SMP
           /*
            * If the callout must migrate try to perform it immediately.
            * If the callout is currently running, just defer the migration
            * to a more appropriate moment.
            */
           if (c->c_cpu != cpu) {
                   if (cc->cc_curr == c) {
                           cc->cc_migration_cpu = cpu;
                           cc->cc_migration_ticks = to_ticks;
                           cc->cc_migration_func = ftn;
                           cc->cc_migration_arg = arg;
                           c->c_flags |= CALLOUT_DFRMIGRATION;
                           CTR5(KTR_CALLOUT,
                       "migration of %p func %p arg %p in %d to %u deferred",
                               c, c->c_func, c->c_arg, to_ticks, cpu);
                           CC_UNLOCK(cc);
                           return (cancelled);
                   }
                   cc = callout_cpu_switch(c, cc, cpu);
           }
   #endif
   
           callout_cc_add(c, cc, to_ticks, ftn, arg, cpu);
           CTR5(KTR_CALLOUT, "%sscheduled %p func %p arg %p in %d",
               cancelled ? "re" : "", c, c->c_func, c->c_arg, to_ticks);
           CC_UNLOCK(cc);
   
           return (cancelled);
   }
   
   /*
    * Common idioms that can be optimized in the future.
    */
   int
   callout_schedule_on(struct callout *c, int to_ticks, int cpu)
   {
           return callout_reset_on(c, to_ticks, c->c_func, c->c_arg, cpu);
   }
   
   int
   callout_schedule(struct callout *c, int to_ticks)
   {
           return callout_reset_on(c, to_ticks, c->c_func, c->c_arg, c->c_cpu);
   }
   
   int
   _callout_stop_safe(c, safe)
           struct  callout *c;
           int     safe;
   {
           struct callout_cpu *cc, *old_cc;
           struct lock_class *class;
           int use_lock, sq_locked;
   
           /*
            * Some old subsystems don't hold Giant while running a callout_stop(),
            * so just discard this check for the moment.
            */
           if (!safe && c->c_lock != NULL) {
                   if (c->c_lock == &Giant.lock_object)
                           use_lock = mtx_owned(&Giant);
                   else {
                           use_lock = 1;
                           class = LOCK_CLASS(c->c_lock);
                           class->lc_assert(c->c_lock, LA_XLOCKED);
                   }
           } else
                   use_lock = 0;
   
           sq_locked = 0;
           old_cc = NULL;
   again:
           cc = callout_lock(c);
   
           /*
            * If the callout was migrating while the callout cpu lock was
            * dropped,  just drop the sleepqueue lock and check the states
            * again.
            */
           if (sq_locked != 0 && cc != old_cc) {
   #ifdef SMP
                   CC_UNLOCK(cc);
                   sleepq_release(&old_cc->cc_waiting);
                   sq_locked = 0;
                   old_cc = NULL;
                   goto again;
   #else
                   panic("migration should not happen");
   #endif
           }
   
           /*
            * If the callout isn't pending, it's not on the queue, so
            * don't attempt to remove it from the queue.  We can try to
            * stop it by other means however.
            */
           if (!(c->c_flags & CALLOUT_PENDING)) {
                   c->c_flags &= ~CALLOUT_ACTIVE;
   
                   /*
                    * If it wasn't on the queue and it isn't the current
                    * callout, then we can't stop it, so just bail.
                    */
                   if (cc->cc_curr != c) {
                           CTR3(KTR_CALLOUT, "failed to stop %p func %p arg %p",
                               c, c->c_func, c->c_arg);
                           CC_UNLOCK(cc);
                           if (sq_locked)
                                   sleepq_release(&cc->cc_waiting);
                           return (0);
                   }
   
                   if (safe) {
                           /*
                            * The current callout is running (or just
                            * about to run) and blocking is allowed, so
                            * just wait for the current invocation to
                            * finish.
                            */
                           while (cc->cc_curr == c) {
   
                                   /*
                                    * Use direct calls to sleepqueue interface
                                    * instead of cv/msleep in order to avoid
                                    * a LOR between cc_lock and sleepqueue
                                    * chain spinlocks.  This piece of code
                                    * emulates a msleep_spin() call actually.
                                    *
                                    * If we already have the sleepqueue chain
                                    * locked, then we can safely block.  If we
                                    * don't already have it locked, however,
                                    * we have to drop the cc_lock to lock
                                    * it.  This opens several races, so we
                                    * restart at the beginning once we have
                                    * both locks.  If nothing has changed, then
                                    * we will end up back here with sq_locked
                                    * set.
                                    */
                                   if (!sq_locked) {
                                           CC_UNLOCK(cc);
                                           sleepq_lock(&cc->cc_waiting);
                                           sq_locked = 1;
                                           old_cc = cc;
                                           goto again;
                                   }
   
                                   /*
                                    * Migration could be cancelled here, but
                                    * as long as it is still not sure when it
                                    * will be packed up, just let softclock()
                                    * take care of it.
                                    */
                                   cc->cc_waiting = 1;
                                   DROP_GIANT();
                                   CC_UNLOCK(cc);
                                   sleepq_add(&cc->cc_waiting,
                                       &cc->cc_lock.lock_object, "codrain",
                                       SLEEPQ_SLEEP, 0);
                                   sleepq_wait(&cc->cc_waiting, 0);
                                   sq_locked = 0;
                                   old_cc = NULL;
   
                                   /* Reacquire locks previously released. */
                                   PICKUP_GIANT();
                                   CC_LOCK(cc);
                           }
                   } else if (use_lock && !cc->cc_cancel) {
                           /*
                            * The current callout is waiting for its
                            * lock which we hold.  Cancel the callout
                            * and return.  After our caller drops the
                            * lock, the callout will be skipped in
                            * softclock().
                           */
                          cc->cc_cancel = 1;
                          CTR3(KTR_CALLOUT, "cancelled %p func %p arg %p",
                              c, c->c_func, c->c_arg);
                          KASSERT(!cc_cme_migrating(cc),
                              ("callout wrongly scheduled for migration"));
                          CC_UNLOCK(cc);
                          KASSERT(!sq_locked, ("sleepqueue chain locked"));
                          return (1);
                  } else if ((c->c_flags & CALLOUT_DFRMIGRATION) != 0) {
                          c->c_flags &= ~CALLOUT_DFRMIGRATION;
                          CTR3(KTR_CALLOUT, "postponing stop %p func %p arg %p",
                              c, c->c_func, c->c_arg);
                          CC_UNLOCK(cc);
                          return (1);
                  }
                  CTR3(KTR_CALLOUT, "failed to stop %p func %p arg %p",
                      c, c->c_func, c->c_arg);
                  CC_UNLOCK(cc);
                  KASSERT(!sq_locked, ("sleepqueue chain still locked"));
                  return (0);
          }
          if (sq_locked)
                  sleepq_release(&cc->cc_waiting);
  
          c->c_flags &= ~(CALLOUT_ACTIVE | CALLOUT_PENDING);
  
          CTR3(KTR_CALLOUT, "cancelled %p func %p arg %p",
              c, c->c_func, c->c_arg);
          if (cc->cc_next == c)
                  cc->cc_next = TAILQ_NEXT(c, c_links.tqe);
          TAILQ_REMOVE(&cc->cc_callwheel[c->c_time & callwheelmask], c,
              c_links.tqe);
          callout_cc_del(c, cc);
  
          CC_UNLOCK(cc);
          return (1);
  }
  
  void
  callout_init(c, mpsafe)
          struct  callout *c;
          int mpsafe;
  {
          bzero(c, sizeof *c);
          if (mpsafe) {
                  c->c_lock = NULL;
                  c->c_flags = CALLOUT_RETURNUNLOCKED;
          } else {
                  c->c_lock = &Giant.lock_object;
                  c->c_flags = 0;
          }
          c->c_cpu = timeout_cpu;
  }
  
  void
  _callout_init_lock(c, lock, flags)
          struct  callout *c;
          struct  lock_object *lock;
          int flags;
  {
          bzero(c, sizeof *c);
          c->c_lock = lock;
          KASSERT((flags & ~(CALLOUT_RETURNUNLOCKED | CALLOUT_SHAREDLOCK)) == 0,
              ("callout_init_lock: bad flags %d", flags));
          KASSERT(lock != NULL || (flags & CALLOUT_RETURNUNLOCKED) == 0,
              ("callout_init_lock: CALLOUT_RETURNUNLOCKED with no lock"));
          KASSERT(lock == NULL || !(LOCK_CLASS(lock)->lc_flags &
              (LC_SPINLOCK | LC_SLEEPABLE)), ("%s: invalid lock class",
              __func__));
          c->c_flags = flags & (CALLOUT_RETURNUNLOCKED | CALLOUT_SHAREDLOCK);
          c->c_cpu = timeout_cpu;
  }
  
  #ifdef APM_FIXUP_CALLTODO
  /* 
   * Adjust the kernel calltodo timeout list.  This routine is used after 
   * an APM resume to recalculate the calltodo timer list values with the 
   * number of hz's we have been sleeping.  The next hardclock() will detect 
   * that there are fired timers and run softclock() to execute them.
   *
   * Please note, I have not done an exhaustive analysis of what code this
   * might break.  I am motivated to have my select()'s and alarm()'s that
   * have expired during suspend firing upon resume so that the applications
   * which set the timer can do the maintanence the timer was for as close
   * as possible to the originally intended time.  Testing this code for a 
   * week showed that resuming from a suspend resulted in 22 to 25 timers 
   * firing, which seemed independant on whether the suspend was 2 hours or
   * 2 days.  Your milage may vary.   - Ken Key <key@cs.utk.edu>
   */
  void
  adjust_timeout_calltodo(time_change)
      struct timeval *time_change;
  {
          register struct callout *p;
          unsigned long delta_ticks;
  
          /* 
           * How many ticks were we asleep?
           * (stolen from tvtohz()).
           */
  
          /* Don't do anything */
          if (time_change->tv_sec < 0)
                  return;
          else if (time_change->tv_sec <= LONG_MAX / 1000000)
                  delta_ticks = (time_change->tv_sec * 1000000 +
                                 time_change->tv_usec + (tick - 1)) / tick + 1;
          else if (time_change->tv_sec <= LONG_MAX / hz)
                  delta_ticks = time_change->tv_sec * hz +
                                (time_change->tv_usec + (tick - 1)) / tick + 1;
          else
                  delta_ticks = LONG_MAX;
  
          if (delta_ticks > INT_MAX)
                  delta_ticks = INT_MAX;
  
          /* 
           * Now rip through the timer calltodo list looking for timers
           * to expire.
           */
  
          /* don't collide with softclock() */
          CC_LOCK(cc);
          for (p = calltodo.c_next; p != NULL; p = p->c_next) {
                  p->c_time -= delta_ticks;
  
                  /* Break if the timer had more time on it than delta_ticks */
                  if (p->c_time > 0)
                          break;
  
                  /* take back the ticks the timer didn't use (p->c_time <= 0) */
                  delta_ticks = -p->c_time;
          }
          CC_UNLOCK(cc);
  
          return;
  }
  #endif /* APM_FIXUP_CALLTODO */

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