sys/kern/kern_mplock.c

   1 /*
   2  * Copyright (c) 2009 The DragonFly Project.  All rights reserved.
   3  *
   4  * This code is derived from software contributed to The DragonFly Project
   5  * by Matthew Dillon <dillon@backplane.com>
   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  *
  11  * 1. Redistributions of source code must retain the above copyright
  12  *    notice, this list of conditions and the following disclaimer.
  13  * 2. Redistributions in binary form must reproduce the above copyright
  14  *    notice, this list of conditions and the following disclaimer in
  15  *    the documentation and/or other materials provided with the
  16  *    distribution.
  17  * 3. Neither the name of The DragonFly Project nor the names of its
  18  *    contributors may be used to endorse or promote products derived
  19  *    from this software without specific, prior written permission.
  20  *
  21  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  22  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  23  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  24  * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
  25  * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
  26  * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
  27  * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  28  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
  29  * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  30  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
  31  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  32  * SUCH DAMAGE.
  33  */
  34
  35 /*
  36  * Helper functions for MP lock acquisition and release.
  37  */
  38
  39 #include <sys/param.h>
  40 #include <sys/systm.h>
  41 #include <sys/kernel.h>
  42 #include <sys/proc.h>
  43 #include <sys/rtprio.h>
  44 #include <sys/queue.h>
  45 #include <sys/sysctl.h>
  46 #include <sys/kthread.h>
  47 #include <machine/cpu.h>
  48 #include <sys/lock.h>
  49 #include <sys/caps.h>
  50 #include <sys/spinlock.h>
  51 #include <sys/ktr.h>
  52
  53 #include <sys/thread2.h>
  54 #include <sys/mplock2.h>
  55 #include <sys/spinlock2.h>
  56
  57 #ifdef SMP
  58 static int chain_mplock = 0;
  59 static int bgl_yield = 10;
  60 static __int64_t mplock_contention_count = 0;
  61
  62 SYSCTL_INT(_lwkt, OID_AUTO, chain_mplock, CTLFLAG_RW, &chain_mplock, 0, "");
  63 SYSCTL_INT(_lwkt, OID_AUTO, bgl_yield_delay, CTLFLAG_RW, &bgl_yield, 0, "");
  64 SYSCTL_QUAD(_lwkt, OID_AUTO, mplock_contention_count, CTLFLAG_RW,
  65         &mplock_contention_count, 0, "spinning due to MPLOCK contention");
  66
  67 /*
  68  * Kernel Trace
  69  */
  70 #if !defined(KTR_GIANT_CONTENTION)
  71 #define KTR_GIANT_CONTENTION    KTR_ALL
  72 #endif
  73
  74 KTR_INFO_MASTER(giant);
  75 KTR_INFO(KTR_GIANT_CONTENTION, giant, beg, 0,
  76         "thread=%p held %s:%-5d  want %s:%-5d",
  77          sizeof(void *) * 3 + sizeof(int) * 2);
  78 KTR_INFO(KTR_GIANT_CONTENTION, giant, end, 1,
  79         "thread=%p held %s:%-5d  want %s:%-5d",
  80          sizeof(void *) * 3 + sizeof(int) * 2);
  81
  82 #define loggiant(name)                                          \
  83         KTR_LOG(giant_ ## name, curthread,                      \
  84                 mp_lock_holder_file, mp_lock_holder_line,       \
  85                 file, line)
  86
  87 int     mp_lock;
  88 int     mp_lock_contention_mask;
  89 const char *mp_lock_holder_file;        /* debugging */
  90 int     mp_lock_holder_line;            /* debugging */
  91
  92 /*
  93  * Sets up the initial MP lock state near the start of the kernel boot
  94  */
  95 void
  96 cpu_get_initial_mplock(void)
  97 {
  98         mp_lock = 0;    /* cpu 0 */
  99         curthread->td_mpcount = 1;
 100 }
 101
 102 /*
 103  * Called when the MP lock could not be trvially acquired.  The caller
 104  * has already bumped td_mpcount.
 105  */
 106 void
 107 _get_mplock_contested(const char *file, int line)
 108 {
 109         globaldata_t gd = mycpu;
 110         int ov;
 111         int nv;
 112
 113         ++mplock_contention_count;
 114         for (;;) {
 115                 ov = mp_lock;
 116                 nv = gd->gd_cpuid;
 117                 if (ov == gd->gd_cpuid)
 118                         break;
 119                 if (ov == -1) {
 120                         if (atomic_cmpset_int(&mp_lock, ov, gd->gd_cpuid))
 121                                 break;
 122                 } else {
 123                         loggiant(beg);
 124                         lwkt_switch();
 125                         loggiant(end);
 126                         KKASSERT(gd->gd_cpuid == mp_lock);
 127                         break;
 128                 }
 129         }
 130 }
 131
 132 /*
 133  * Called if td_mpcount went negative or if td_mpcount is 0 and we were
 134  * unable to release the MP lock.  Handles sanity checks and conflicts.
 135  *
 136  * It is possible for the inline release to have raced an interrupt which
 137  * get/rel'd the MP lock, causing the inline's cmpset to fail.  If this
 138  * case occurs mp_lock will either already be in a released state or it
 139  * will have already been acquired by another cpu.
 140  */
 141 void
 142 _rel_mplock_contested(void)
 143 {
 144         globaldata_t gd = mycpu;
 145         int ov;
 146
 147         KKASSERT(gd->gd_curthread->td_mpcount >= 0);
 148         for (;;) {
 149                 ov = mp_lock;
 150                 if (ov != gd->gd_cpuid)
 151                         break;
 152                 if (atomic_cmpset_int(&mp_lock, ov, -1))
 153                         break;
 154         }
 155 }
 156
 157 /*
 158  * Called when try_mplock() fails.
 159  *
 160  * The inline bumped td_mpcount so we have to undo it.
 161  *
 162  * It is possible to race an interrupt which acquired and released the
 163  * MP lock.  When combined with the td_mpcount decrement we do the MP lock
 164  * can wind up in any state and possibly not even owned by us.
 165  *
 166  * It is also possible for this function to be called even if td_mpcount > 1
 167  * if someone bumped it and raced an interrupt which then called try_mpock().
 168  */
 169 void
 170 _try_mplock_contested(const char *file, int line)
 171 {
 172         globaldata_t gd = mycpu;
 173         thread_t td = gd->gd_curthread;
 174         int ov;
 175
 176         --td->td_mpcount;
 177         KKASSERT(td->td_mpcount >= 0);
 178         ++mplock_contention_count;
 179
 180         for (;;) {
 181                 ov = mp_lock;
 182                 if (ov != gd->gd_cpuid)
 183                         break;
 184                 if (atomic_cmpset_int(&mp_lock, ov, -1))
 185                         break;
 186         }
 187 }
 188
 189 /*
 190  * Called when cpu_try_mplock() fails.
 191  *
 192  * The inline did not touch td_mpcount so we do not either.
 193  */
 194 void
 195 _cpu_try_mplock_contested(const char *file, int line)
 196 {
 197         ++mplock_contention_count;
 198 }
 199
 200 /*
 201  * Temporarily yield the MP lock.  This is part of lwkt_user_yield()
 202  * which is kinda hackish.
 203  */
 204 void
 205 yield_mplock(thread_t td)
 206 {
 207         int savecnt;
 208
 209         savecnt = td->td_mpcount;
 210         td->td_mpcount = 1;
 211         rel_mplock();
 212         DELAY(bgl_yield);
 213         get_mplock();
 214         td->td_mpcount = savecnt;
 215 }
 216
 217 #if 0
 218
 219 /*
 220  * The rel_mplock() code will call this function after releasing the
 221  * last reference on the MP lock if mp_lock_contention_mask is non-zero.
 222  *
 223  * We then chain an IPI to a single other cpu potentially needing the
 224  * lock.  This is a bit heuristical and we can wind up with IPIs flying
 225  * all over the place.
 226  */
 227 static void lwkt_mp_lock_uncontested_remote(void *arg __unused);
 228
 229 void
 230 lwkt_mp_lock_uncontested(void)
 231 {
 232     globaldata_t gd;
 233     globaldata_t dgd;
 234     cpumask_t mask;
 235     cpumask_t tmpmask;
 236     int cpuid;
 237
 238     if (chain_mplock) {
 239         gd = mycpu;
 240         clr_mplock_contention_mask(gd);
 241         mask = mp_lock_contention_mask;
 242         tmpmask = ~((1 << gd->gd_cpuid) - 1);
 243
 244         if (mask) {
 245             if (mask & tmpmask)
 246                     cpuid = bsfl(mask & tmpmask);
 247             else
 248                     cpuid = bsfl(mask);
 249             atomic_clear_int(&mp_lock_contention_mask, 1 << cpuid);
 250             dgd = globaldata_find(cpuid);
 251             lwkt_send_ipiq(dgd, lwkt_mp_lock_uncontested_remote, NULL);
 252         }
 253     }
 254 }
 255
 256 /*
 257  * The idea is for this IPI to interrupt a potentially lower priority
 258  * thread, such as a user thread, to allow the scheduler to reschedule
 259  * a higher priority kernel thread that needs the MP lock.
 260  *
 261  * For now we set the LWKT reschedule flag which generates an AST in
 262  * doreti, though theoretically it is also possible to possibly preempt
 263  * here if the underlying thread was operating in user mode.  Nah.
 264  */
 265 static void
 266 lwkt_mp_lock_uncontested_remote(void *arg __unused)
 267 {
 268         need_lwkt_resched();
 269 }
 270
 271 #endif
 272
 273 #endif  /* SMP */