/* Copyright (c) 2006, Google Inc. * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are * met: * * * Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * * 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. * * Neither the name of Google Inc. 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 COPYRIGHT HOLDERS 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 COPYRIGHT * OWNER 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. * * --- * Author: Sanjay Ghemawat */ #include <config.h> #include "base/spinlock.h" #include "base/synchronization_profiling.h" #include "base/spinlock_internal.h" #include "base/cycleclock.h" #include "base/sysinfo.h" /* for NumCPUs() */ // NOTE on the Lock-state values: // // kSpinLockFree represents the unlocked state // kSpinLockHeld represents the locked state with no waiters // // Values greater than kSpinLockHeld represent the locked state with waiters, // where the value is the time the current lock holder had to // wait before obtaining the lock. The kSpinLockSleeper state is a special // "locked with waiters" state that indicates that a sleeper needs to // be woken, but the thread that just released the lock didn't wait. static int adaptive_spin_count = 0; const base::LinkerInitialized SpinLock::LINKER_INITIALIZED = base::LINKER_INITIALIZED; namespace { struct SpinLock_InitHelper { SpinLock_InitHelper() { // On multi-cpu machines, spin for longer before yielding // the processor or sleeping. Reduces idle time significantly. if (NumCPUs() > 1) { adaptive_spin_count = 1000; } } }; // Hook into global constructor execution: // We do not do adaptive spinning before that, // but nothing lock-intensive should be going on at that time. static SpinLock_InitHelper init_helper; } // unnamed namespace // Monitor the lock to see if its value changes within some time period // (adaptive_spin_count loop iterations). A timestamp indicating // when the thread initially started waiting for the lock is passed in via // the initial_wait_timestamp value. The total wait time in cycles for the // lock is returned in the wait_cycles parameter. The last value read // from the lock is returned from the method. Atomic32 SpinLock::SpinLoop(int64 initial_wait_timestamp, Atomic32* wait_cycles) { int c = adaptive_spin_count; while (base::subtle::NoBarrier_Load(&lockword_) != kSpinLockFree && --c > 0) { } Atomic32 spin_loop_wait_cycles = CalculateWaitCycles(initial_wait_timestamp); Atomic32 lock_value = base::subtle::Acquire_CompareAndSwap(&lockword_, kSpinLockFree, spin_loop_wait_cycles); *wait_cycles = spin_loop_wait_cycles; return lock_value; } void SpinLock::SlowLock() { // The lock was not obtained initially, so this thread needs to wait for // it. Record the current timestamp in the local variable wait_start_time // so the total wait time can be stored in the lockword once this thread // obtains the lock. int64 wait_start_time = CycleClock::Now(); Atomic32 wait_cycles; Atomic32 lock_value = SpinLoop(wait_start_time, &wait_cycles); int lock_wait_call_count = 0; while (lock_value != kSpinLockFree) { // If the lock is currently held, but not marked as having a sleeper, mark // it as having a sleeper. if (lock_value == kSpinLockHeld) { // Here, just "mark" that the thread is going to sleep. Don't store the // lock wait time in the lock as that will cause the current lock // owner to think it experienced contention. lock_value = base::subtle::Acquire_CompareAndSwap(&lockword_, kSpinLockHeld, kSpinLockSleeper); if (lock_value == kSpinLockHeld) { // Successfully transitioned to kSpinLockSleeper. Pass // kSpinLockSleeper to the SpinLockWait routine to properly indicate // the last lock_value observed. lock_value = kSpinLockSleeper; } else if (lock_value == kSpinLockFree) { // Lock is free again, so try and aquire it before sleeping. The // new lock state will be the number of cycles this thread waited if // this thread obtains the lock. lock_value = base::subtle::Acquire_CompareAndSwap(&lockword_, kSpinLockFree, wait_cycles); continue; // skip the delay at the end of the loop } } // Wait for an OS specific delay. base::internal::SpinLockDelay(&lockword_, lock_value, ++lock_wait_call_count); // Spin again after returning from the wait routine to give this thread // some chance of obtaining the lock. lock_value = SpinLoop(wait_start_time, &wait_cycles); } } // The wait time for contentionz lock profiling must fit into 32 bits. // However, the lower 32-bits of the cycle counter wrap around too quickly // with high frequency processors, so a right-shift by 7 is performed to // quickly divide the cycles by 128. Using these 32 bits, reduces the // granularity of time measurement to 128 cycles, and loses track // of wait time for waits greater than 109 seconds on a 5 GHz machine // [(2^32 cycles/5 Ghz)*128 = 109.95 seconds]. Waits this long should be // very rare and the reduced granularity should not be an issue given // processors in the Google fleet operate at a minimum of one billion // cycles/sec. enum { PROFILE_TIMESTAMP_SHIFT = 7 }; void SpinLock::SlowUnlock(uint64 wait_cycles) { base::internal::SpinLockWake(&lockword_, false); // wake waiter if necessary // Collect contentionz profile info, expanding the wait_cycles back out to // the full value. If wait_cycles is <= kSpinLockSleeper, then no wait // was actually performed, so don't record the wait time. Note, that the // CalculateWaitCycles method adds in kSpinLockSleeper cycles // unconditionally to guarantee the wait time is not kSpinLockFree or // kSpinLockHeld. The adding in of these small number of cycles may // overestimate the contention by a slight amount 50% of the time. However, // if this code tried to correct for that addition by subtracting out the // kSpinLockSleeper amount that would underestimate the contention slightly // 50% of the time. Both ways get the wrong answer, so the code // overestimates to be more conservative. Overestimating also makes the code // a little simpler. // if (wait_cycles > kSpinLockSleeper) { base::SubmitSpinLockProfileData(this, wait_cycles << PROFILE_TIMESTAMP_SHIFT); } } inline int32 SpinLock::CalculateWaitCycles(int64 wait_start_time) { int32 wait_cycles = ((CycleClock::Now() - wait_start_time) >> PROFILE_TIMESTAMP_SHIFT); // The number of cycles waiting for the lock is used as both the // wait_cycles and lock value, so it can't be kSpinLockFree or // kSpinLockHeld. Make sure the value returned is at least // kSpinLockSleeper. wait_cycles |= kSpinLockSleeper; return wait_cycles; }