kern_spinlock.c

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    1 /*
    2  * Copyright (c) 2005 Jeffrey M. Hsu.  All rights reserved.
    3  *
    4  * This code is derived from software contributed to The DragonFly Project
    5  * by Jeffrey M. Hsu. and Matthew Dillon
    6  *
    7  * Redistribution and use in source and binary forms, with or without
    8  * modification, are permitted provided that the following conditions
    9  * are met:
   10  * 1. Redistributions of source code must retain the above copyright
   11  *    notice, this list of conditions and the following disclaimer.
   12  * 2. Redistributions in binary form must reproduce the above copyright
   13  *    notice, this list of conditions and the following disclaimer in the
   14  *    documentation and/or other materials provided with the distribution.
   15  * 3. Neither the name of The DragonFly Project nor the names of its
   16  *    contributors may be used to endorse or promote products derived
   17  *    from this software without specific, prior written permission.
   18  *
   19  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   20  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   21  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   22  * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
   23  * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   24  * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
   25  * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   26  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
   27  * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
   28  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
   29  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   30  * SUCH DAMAGE.
   31  */
   32 
   33 /*
   34  * The implementation is designed to avoid looping when compatible operations
   35  * are executed.
   36  *
   37  * To acquire a spinlock we first increment counta.  Then we check if counta
   38  * meets our requirements.  For an exclusive spinlock it must be 1, of a
   39  * shared spinlock it must either be 1 or the SHARED_SPINLOCK bit must be set.
   40  *
   41  * Shared spinlock failure case: Decrement the count, loop until we can
   42  * transition from 0 to SHARED_SPINLOCK|1, or until we find SHARED_SPINLOCK
   43  * is set and increment the count.
   44  *
   45  * Exclusive spinlock failure case: While maintaining the count, clear the
   46  * SHARED_SPINLOCK flag unconditionally.  Then use an atomic add to transfer
   47  * the count from the low bits to the high bits of counta.  Then loop until
   48  * all low bits are 0.  Once the low bits drop to 0 we can transfer the
   49  * count back with an atomic_cmpset_int(), atomically, and return.
   50  */
   51 #include <sys/param.h>
   52 #include <sys/systm.h>
   53 #include <sys/types.h>
   54 #include <sys/kernel.h>
   55 #include <sys/sysctl.h>
   56 #ifdef INVARIANTS
   57 #include <sys/proc.h>
   58 #endif
   59 #include <sys/priv.h>
   60 #include <machine/atomic.h>
   61 #include <machine/cpu.h>
   62 #include <machine/cpufunc.h>
   63 #include <machine/specialreg.h>
   64 #include <machine/clock.h>
   65 #include <sys/spinlock.h>
   66 #include <sys/spinlock2.h>
   67 #include <sys/ktr.h>
   68 
   69 #ifdef _KERNEL_VIRTUAL
   70 #include <pthread.h>
   71 #endif
   72 
   73 struct spinlock pmap_spin = SPINLOCK_INITIALIZER(pmap_spin);
   74 
   75 struct indefinite_info {
   76         sysclock_t      base;
   77         int             secs;
   78         const char      *ident;
   79 };
   80 
   81 /*
   82  * Kernal Trace
   83  */
   84 #if !defined(KTR_SPIN_CONTENTION)
   85 #define KTR_SPIN_CONTENTION     KTR_ALL
   86 #endif
   87 #define SPIN_STRING     "spin=%p type=%c"
   88 #define SPIN_ARG_SIZE   (sizeof(void *) + sizeof(int))
   89 
   90 KTR_INFO_MASTER(spin);
   91 #if 0
   92 KTR_INFO(KTR_SPIN_CONTENTION, spin, beg, 0, SPIN_STRING, SPIN_ARG_SIZE);
   93 KTR_INFO(KTR_SPIN_CONTENTION, spin, end, 1, SPIN_STRING, SPIN_ARG_SIZE);
   94 #endif
   95 
   96 #define logspin(name, spin, type)                       \
   97         KTR_LOG(spin_ ## name, spin, type)
   98 
   99 #ifdef INVARIANTS
  100 static int spin_lock_test_mode;
  101 #endif
  102 
  103 #ifdef DEBUG_LOCKS_LATENCY
  104 
  105 static long spinlocks_add_latency;
  106 SYSCTL_LONG(_debug, OID_AUTO, spinlocks_add_latency, CTLFLAG_RW,
  107     &spinlocks_add_latency, 0,
  108     "Add spinlock latency");
  109 
  110 #endif
  111 
  112 
  113 /*
  114  * We need a fairly large pool to avoid contention on large SMP systems,
  115  * particularly multi-chip systems.
  116  */
  117 /*#define SPINLOCK_NUM_POOL     8101*/
  118 #define SPINLOCK_NUM_POOL       8192
  119 #define SPINLOCK_NUM_POOL_MASK  (SPINLOCK_NUM_POOL - 1)
  120 
  121 static __cachealign struct {
  122         struct spinlock spin;
  123         char filler[32 - sizeof(struct spinlock)];
  124 } pool_spinlocks[SPINLOCK_NUM_POOL];
  125 
  126 static int spin_indefinite_check(struct spinlock *spin,
  127                                   struct indefinite_info *info);
  128 
  129 /*
  130  * We contested due to another exclusive lock holder.  We lose.
  131  *
  132  * We have to unwind the attempt and may acquire the spinlock
  133  * anyway while doing so.  countb was incremented on our behalf.
  134  */
  135 int
  136 spin_trylock_contested(struct spinlock *spin)
  137 {
  138         globaldata_t gd = mycpu;
  139 
  140         /*atomic_add_int(&spin->counta, -1);*/
  141         --gd->gd_spinlocks;
  142         --gd->gd_curthread->td_critcount;
  143         return (FALSE);
  144 }
  145 
  146 /*
  147  * The spin_lock() inline was unable to acquire the lock.
  148  *
  149  * atomic_swap_int() is the absolute fastest spinlock instruction, at
  150  * least on multi-socket systems.  All instructions seem to be about
  151  * the same on single-socket multi-core systems.  However, atomic_swap_int()
  152  * does not result in an even distribution of successful acquisitions.
  153  *
  154  * UNFORTUNATELY we cannot really use atomic_swap_int() when also implementing
  155  * shared spin locks, so as we do a better job removing contention we've
  156  * moved to atomic_cmpset_int() to be able handle multiple states.
  157  *
  158  * Another problem we have is that (at least on the 48-core opteron we test
  159  * with) having all 48 cores contesting the same spin lock reduces
  160  * performance to around 600,000 ops/sec, verses millions when fewer cores
  161  * are going after the same lock.
  162  *
  163  * Backoff algorithms can create even worse starvation problems, and don't
  164  * really improve performance when a lot of cores are contending.
  165  *
  166  * Our solution is to allow the data cache to lazy-update by reading it
  167  * non-atomically and only attempting to acquire the lock if the lazy read
  168  * looks good.  This effectively limits cache bus bandwidth.  A cpu_pause()
  169  * (for intel/amd anyhow) is not strictly needed as cache bus resource use
  170  * is governed by the lazy update.
  171  *
  172  * WARNING!!!!  Performance matters here, by a huge margin.
  173  *
  174  *      48-core test with pre-read / -j 48 no-modules kernel compile
  175  *      with fanned-out inactive and active queues came in at 55 seconds.
  176  *
  177  *      48-core test with pre-read / -j 48 no-modules kernel compile
  178  *      came in at 75 seconds.  Without pre-read it came in at 170 seconds.
  179  *
  180  *      4-core test with pre-read / -j 48 no-modules kernel compile
  181  *      came in at 83 seconds.  Without pre-read it came in at 83 seconds
  182  *      as well (no difference).
  183  */
  184 void
  185 _spin_lock_contested(struct spinlock *spin, const char *ident)
  186 {
  187         struct indefinite_info info = { 0, 0, ident };
  188         int i;
  189 
  190         /*
  191          * Transfer our count to the high bits, then loop until we can
  192          * acquire the low counter (== 1).  No new shared lock can be
  193          * acquired while we hold the EXCLWAIT bits.
  194          *
  195          * Force any existing shared locks to exclusive.  The shared unlock
  196          * understands that this may occur.
  197          */
  198         atomic_add_int(&spin->counta, SPINLOCK_EXCLWAIT - 1);
  199         atomic_clear_int(&spin->counta, SPINLOCK_SHARED);
  200 
  201 #ifdef DEBUG_LOCKS_LATENCY
  202         long j;
  203         for (j = spinlocks_add_latency; j > 0; --j)
  204                 cpu_ccfence();
  205 #endif
  206 #if defined(INVARIANTS)
  207         if (spin_lock_test_mode > 10 &&
  208             spin->countb > spin_lock_test_mode &&
  209             (spin_lock_test_mode & 0xFF) == mycpu->gd_cpuid) {
  210                 spin->countb = 0;
  211                 print_backtrace(-1);
  212         }
  213         ++spin->countb;
  214 #endif
  215         i = 0;
  216 
  217         /*logspin(beg, spin, 'w');*/
  218         for (;;) {
  219                 /*
  220                  * If the low bits are zero, try to acquire the exclusive lock
  221                  * by transfering our high bit counter to the low bits.
  222                  *
  223                  * NOTE: Reading spin->counta prior to the swap is extremely
  224                  *       important on multi-chip/many-core boxes.  On 48-core
  225                  *       this one change improves fully concurrent all-cores
  226                  *       compiles by 100% or better.
  227                  *
  228                  *       I can't emphasize enough how important the pre-read
  229                  *       is in preventing hw cache bus armageddon on
  230                  *       multi-chip systems.  And on single-chip/multi-core
  231                  *       systems it just doesn't hurt.
  232                  */
  233                 uint32_t ovalue = spin->counta;
  234                 cpu_ccfence();
  235                 if ((ovalue & (SPINLOCK_EXCLWAIT - 1)) == 0 &&
  236                     atomic_cmpset_int(&spin->counta, ovalue,
  237                                       (ovalue - SPINLOCK_EXCLWAIT) | 1)) {
  238                         break;
  239                 }
  240                 if ((++i & 0x7F) == 0x7F) {
  241                         mycpu->gd_cnt.v_lock_name[0] = 'X';
  242                         strncpy(mycpu->gd_cnt.v_lock_name + 1,
  243                                 ident,
  244                                 sizeof(mycpu->gd_cnt.v_lock_name) - 2);
  245                         ++mycpu->gd_cnt.v_lock_colls;
  246 #if defined(INVARIANTS)
  247                         ++spin->countb;
  248 #endif
  249                         if (spin_indefinite_check(spin, &info))
  250                                 break;
  251                 }
  252 #ifdef _KERNEL_VIRTUAL
  253                 pthread_yield();
  254 #endif
  255         }
  256         /*logspin(end, spin, 'w');*/
  257 }
  258 
  259 /*
  260  * Shared spinlock attempt was contested.
  261  *
  262  * The caller has not modified counta.
  263  */
  264 void
  265 _spin_lock_shared_contested(struct spinlock *spin, const char *ident)
  266 {
  267         struct indefinite_info info = { 0, 0, ident };
  268         int i;
  269 
  270 #ifdef DEBUG_LOCKS_LATENCY
  271         long j;
  272         for (j = spinlocks_add_latency; j > 0; --j)
  273                 cpu_ccfence();
  274 #endif
  275 #if defined(INVARIANTS)
  276         if (spin_lock_test_mode > 10 &&
  277             spin->countb > spin_lock_test_mode &&
  278             (spin_lock_test_mode & 0xFF) == mycpu->gd_cpuid) {
  279                 spin->countb = 0;
  280                 print_backtrace(-1);
  281         }
  282         ++spin->countb;
  283 #endif
  284         i = 0;
  285 
  286         /*logspin(beg, spin, 'w');*/
  287         for (;;) {
  288                 /*
  289                  * Loop until we can acquire the shared spinlock.  Note that
  290                  * the low bits can be zero while the high EXCLWAIT bits are
  291                  * non-zero.  In this situation exclusive requesters have
  292                  * priority (otherwise shared users on multiple cpus can hog
  293                  * the spinlnock).
  294                  *
  295                  * NOTE: Reading spin->counta prior to the swap is extremely
  296                  *       important on multi-chip/many-core boxes.  On 48-core
  297                  *       this one change improves fully concurrent all-cores
  298                  *       compiles by 100% or better.
  299                  *
  300                  *       I can't emphasize enough how important the pre-read
  301                  *       is in preventing hw cache bus armageddon on
  302                  *       multi-chip systems.  And on single-chip/multi-core
  303                  *       systems it just doesn't hurt.
  304                  */
  305                 uint32_t ovalue = spin->counta;
  306 
  307                 cpu_ccfence();
  308                 if (ovalue == 0) {
  309                         if (atomic_cmpset_int(&spin->counta, 0,
  310                                               SPINLOCK_SHARED | 1))
  311                                 break;
  312                 } else if (ovalue & SPINLOCK_SHARED) {
  313                         if (atomic_cmpset_int(&spin->counta, ovalue,
  314                                               ovalue + 1))
  315                                 break;
  316                 }
  317                 if ((++i & 0x7F) == 0x7F) {
  318                         mycpu->gd_cnt.v_lock_name[0] = 'S';
  319                         strncpy(mycpu->gd_cnt.v_lock_name + 1,
  320                                 ident,
  321                                 sizeof(mycpu->gd_cnt.v_lock_name) - 2);
  322                         ++mycpu->gd_cnt.v_lock_colls;
  323 #if defined(INVARIANTS)
  324                         ++spin->countb;
  325 #endif
  326                         if (spin_indefinite_check(spin, &info))
  327                                 break;
  328                 }
  329 #ifdef _KERNEL_VIRTUAL
  330                 pthread_yield();
  331 #endif
  332         }
  333         /*logspin(end, spin, 'w');*/
  334 }
  335 
  336 /*
  337  * Pool functions (SHARED SPINLOCKS NOT SUPPORTED)
  338  */
  339 static __inline int
  340 _spin_pool_hash(void *ptr)
  341 {
  342         int i;
  343 
  344         i = ((int)(uintptr_t) ptr >> 5) ^ ((int)(uintptr_t)ptr >> 12);
  345         i &= SPINLOCK_NUM_POOL_MASK;
  346         return (i);
  347 }
  348 
  349 void
  350 _spin_pool_lock(void *chan, const char *ident)
  351 {
  352         struct spinlock *sp;
  353 
  354         sp = &pool_spinlocks[_spin_pool_hash(chan)].spin;
  355         _spin_lock(sp, ident);
  356 }
  357 
  358 void
  359 _spin_pool_unlock(void *chan)
  360 {
  361         struct spinlock *sp;
  362 
  363         sp = &pool_spinlocks[_spin_pool_hash(chan)].spin;
  364         spin_unlock(sp);
  365 }
  366 
  367 
  368 static
  369 int
  370 spin_indefinite_check(struct spinlock *spin, struct indefinite_info *info)
  371 {
  372         sysclock_t count;
  373 
  374         cpu_spinlock_contested();
  375 
  376         count = sys_cputimer->count();
  377         if (info->secs == 0) {
  378                 info->base = count;
  379                 ++info->secs;
  380         } else if (count - info->base > sys_cputimer->freq) {
  381                 kprintf("spin_lock: %s(%p), indefinite wait (%d secs)!\n",
  382                         info->ident, spin, info->secs);
  383                 info->base = count;
  384                 ++info->secs;
  385                 if (panicstr)
  386                         return (TRUE);
  387 #if defined(INVARIANTS)
  388                 if (spin_lock_test_mode) {
  389                         print_backtrace(-1);
  390                         return (TRUE);
  391                 }
  392 #endif
  393 #if defined(INVARIANTS)
  394                 if (info->secs == 11)
  395                         print_backtrace(-1);
  396 #endif
  397                 if (info->secs == 60)
  398                         panic("spin_lock: %s(%p), indefinite wait!",
  399                               info->ident, spin);
  400         }
  401         return (FALSE);
  402 }
  403 
  404 /*
  405  * If INVARIANTS is enabled various spinlock timing tests can be run
  406  * by setting debug.spin_lock_test:
  407  *
  408  *      1       Test the indefinite wait code
  409  *      2       Time the best-case exclusive lock overhead (spin_test_count)
  410  *      3       Time the best-case shared lock overhead (spin_test_count)
  411  */
  412 
  413 #ifdef INVARIANTS
  414 
  415 static int spin_test_count = 10000000;
  416 SYSCTL_INT(_debug, OID_AUTO, spin_test_count, CTLFLAG_RW, &spin_test_count, 0,
  417     "Number of iterations to use for spinlock wait code test");
  418 
  419 static int
  420 sysctl_spin_lock_test(SYSCTL_HANDLER_ARGS)
  421 {
  422         struct spinlock spin;
  423         int error;
  424         int value = 0;
  425         int i;
  426 
  427         if ((error = priv_check(curthread, PRIV_ROOT)) != 0)
  428                 return (error);
  429         if ((error = SYSCTL_IN(req, &value, sizeof(value))) != 0)
  430                 return (error);
  431 
  432         /*
  433          * Indefinite wait test
  434          */
  435         if (value == 1) {
  436                 spin_init(&spin);
  437                 spin_lock(&spin);       /* force an indefinite wait */
  438                 spin_lock_test_mode = 1;
  439                 spin_lock(&spin);
  440                 spin_unlock(&spin);     /* Clean up the spinlock count */
  441                 spin_unlock(&spin);
  442                 spin_lock_test_mode = 0;
  443         }
  444 
  445         /*
  446          * Time best-case exclusive spinlocks
  447          */
  448         if (value == 2) {
  449                 globaldata_t gd = mycpu;
  450 
  451                 spin_init(&spin);
  452                 for (i = spin_test_count; i > 0; --i) {
  453                     _spin_lock_quick(gd, &spin, "test");
  454                     spin_unlock_quick(gd, &spin);
  455                 }
  456         }
  457 
  458         return (0);
  459 }
  460 
  461 SYSCTL_PROC(_debug, KERN_PROC_ALL, spin_lock_test, CTLFLAG_RW|CTLTYPE_INT,
  462         0, 0, sysctl_spin_lock_test, "I", "Test spinlock wait code");
  463 
  464 #endif  /* INVARIANTS */
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FreeBSD/Linux Kernel Cross Reference sys/kern/kern_spinlock.c

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