third_party/tcmalloc/chromium/src/base/spinlock.cc

   1 /* Copyright (c) 2006, Google Inc.
   2  * All rights reserved.
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions are
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   7  *
   8  *     * Redistributions of source code must retain the above copyright
   9  * notice, this list of conditions and the following disclaimer.
  10  *     * Redistributions in binary form must reproduce the above
  11  * copyright notice, this list of conditions and the following disclaimer
  12  * in the documentation and/or other materials provided with the
  13  * distribution.
  14  *     * Neither the name of Google Inc. nor the names of its
  15  * contributors may be used to endorse or promote products derived from
  16  * this software without specific prior written permission.
  17  *
  18  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  19  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  20  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  21  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  22  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  23  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  24  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  25  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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  27  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  28  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  29  *
  30  * ---
  31  * Author: Sanjay Ghemawat
  32  */
  33
  34 #include <config.h>
  35 #include "base/spinlock.h"
  36 #include "base/synchronization_profiling.h"
  37 #include "base/spinlock_internal.h"
  38 #include "base/cycleclock.h"
  39 #include "base/sysinfo.h"   /* for NumCPUs() */
  40
  41 // NOTE on the Lock-state values:
  42 //
  43 //   kSpinLockFree represents the unlocked state
  44 //   kSpinLockHeld represents the locked state with no waiters
  45 //
  46 // Values greater than kSpinLockHeld represent the locked state with waiters,
  47 // where the value is the time the current lock holder had to
  48 // wait before obtaining the lock.  The kSpinLockSleeper state is a special
  49 // "locked with waiters" state that indicates that a sleeper needs to
  50 // be woken, but the thread that just released the lock didn't wait.
  51
  52 static int adaptive_spin_count = 0;
  53
  54 const base::LinkerInitialized SpinLock::LINKER_INITIALIZED =
  55     base::LINKER_INITIALIZED;
  56
  57 namespace {
  58 struct SpinLock_InitHelper {
  59   SpinLock_InitHelper() {
  60     // On multi-cpu machines, spin for longer before yielding
  61     // the processor or sleeping.  Reduces idle time significantly.
  62     if (NumCPUs() > 1) {
  63       adaptive_spin_count = 1000;
  64     }
  65   }
  66 };
  67
  68 // Hook into global constructor execution:
  69 // We do not do adaptive spinning before that,
  70 // but nothing lock-intensive should be going on at that time.
  71 static SpinLock_InitHelper init_helper;
  72
  73 }  // unnamed namespace
  74
  75 // Monitor the lock to see if its value changes within some time period
  76 // (adaptive_spin_count loop iterations).  A timestamp indicating
  77 // when the thread initially started waiting for the lock is passed in via
  78 // the initial_wait_timestamp value.  The total wait time in cycles for the
  79 // lock is returned in the wait_cycles parameter.  The last value read
  80 // from the lock is returned from the method.
  81 Atomic32 SpinLock::SpinLoop(int64 initial_wait_timestamp,
  82                             Atomic32* wait_cycles) {
  83   int c = adaptive_spin_count;
  84   while (base::subtle::NoBarrier_Load(&lockword_) != kSpinLockFree && --c > 0) {
  85   }
  86   Atomic32 spin_loop_wait_cycles = CalculateWaitCycles(initial_wait_timestamp);
  87   Atomic32 lock_value =
  88       base::subtle::Acquire_CompareAndSwap(&lockword_, kSpinLockFree,
  89                                            spin_loop_wait_cycles);
  90   *wait_cycles = spin_loop_wait_cycles;
  91   return lock_value;
  92 }
  93
  94 void SpinLock::SlowLock() {
  95   // The lock was not obtained initially, so this thread needs to wait for
  96   // it.  Record the current timestamp in the local variable wait_start_time
  97   // so the total wait time can be stored in the lockword once this thread
  98   // obtains the lock.
  99   int64 wait_start_time = CycleClock::Now();
 100   Atomic32 wait_cycles;
 101   Atomic32 lock_value = SpinLoop(wait_start_time, &wait_cycles);
 102
 103   int lock_wait_call_count = 0;
 104   while (lock_value != kSpinLockFree) {
 105     // If the lock is currently held, but not marked as having a sleeper, mark
 106     // it as having a sleeper.
 107     if (lock_value == kSpinLockHeld) {
 108       // Here, just "mark" that the thread is going to sleep.  Don't store the
 109       // lock wait time in the lock as that will cause the current lock
 110       // owner to think it experienced contention.
 111       lock_value = base::subtle::Acquire_CompareAndSwap(&lockword_,
 112                                                         kSpinLockHeld,
 113                                                         kSpinLockSleeper);
 114       if (lock_value == kSpinLockHeld) {
 115         // Successfully transitioned to kSpinLockSleeper.  Pass
 116         // kSpinLockSleeper to the SpinLockWait routine to properly indicate
 117         // the last lock_value observed.
 118         lock_value = kSpinLockSleeper;
 119       } else if (lock_value == kSpinLockFree) {
 120         // Lock is free again, so try and aquire it before sleeping.  The
 121         // new lock state will be the number of cycles this thread waited if
 122         // this thread obtains the lock.
 123         lock_value = base::subtle::Acquire_CompareAndSwap(&lockword_,
 124                                                           kSpinLockFree,
 125                                                           wait_cycles);
 126         continue;  // skip the delay at the end of the loop
 127       }
 128     }
 129
 130     // Wait for an OS specific delay.
 131     base::internal::SpinLockDelay(&lockword_, lock_value,
 132                                   ++lock_wait_call_count);
 133     // Spin again after returning from the wait routine to give this thread
 134     // some chance of obtaining the lock.
 135     lock_value = SpinLoop(wait_start_time, &wait_cycles);
 136   }
 137 }
 138
 139 // The wait time for contentionz lock profiling must fit into 32 bits.
 140 // However, the lower 32-bits of the cycle counter wrap around too quickly
 141 // with high frequency processors, so a right-shift by 7 is performed to
 142 // quickly divide the cycles by 128.  Using these 32 bits, reduces the
 143 // granularity of time measurement to 128 cycles, and loses track
 144 // of wait time for waits greater than 109 seconds on a 5 GHz machine
 145 // [(2^32 cycles/5 Ghz)*128 = 109.95 seconds]. Waits this long should be
 146 // very rare and the reduced granularity should not be an issue given
 147 // processors in the Google fleet operate at a minimum of one billion
 148 // cycles/sec.
 149 enum { PROFILE_TIMESTAMP_SHIFT = 7 };
 150
 151 void SpinLock::SlowUnlock(uint64 wait_cycles) {
 152   base::internal::SpinLockWake(&lockword_, false);  // wake waiter if necessary
 153
 154   // Collect contentionz profile info, expanding the wait_cycles back out to
 155   // the full value.  If wait_cycles is <= kSpinLockSleeper, then no wait
 156   // was actually performed, so don't record the wait time.  Note, that the
 157   // CalculateWaitCycles method adds in kSpinLockSleeper cycles
 158   // unconditionally to guarantee the wait time is not kSpinLockFree or
 159   // kSpinLockHeld.  The adding in of these small number of cycles may
 160   // overestimate the contention by a slight amount 50% of the time.  However,
 161   // if this code tried to correct for that addition by subtracting out the
 162   // kSpinLockSleeper amount that would underestimate the contention slightly
 163   // 50% of the time.  Both ways get the wrong answer, so the code
 164   // overestimates to be more conservative. Overestimating also makes the code
 165   // a little simpler.
 166   //
 167   if (wait_cycles > kSpinLockSleeper) {
 168     base::SubmitSpinLockProfileData(this,
 169                                     wait_cycles << PROFILE_TIMESTAMP_SHIFT);
 170   }
 171 }
 172
 173 inline int32 SpinLock::CalculateWaitCycles(int64 wait_start_time) {
 174   int32 wait_cycles = ((CycleClock::Now() - wait_start_time) >>
 175                        PROFILE_TIMESTAMP_SHIFT);
 176   // The number of cycles waiting for the lock is used as both the
 177   // wait_cycles and lock value, so it can't be kSpinLockFree or
 178   // kSpinLockHeld.  Make sure the value returned is at least
 179   // kSpinLockSleeper.
 180   wait_cycles |= kSpinLockSleeper;
 181   return wait_cycles;
 182 }