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Diffstat (limited to 'absl/base/internal/spinlock.cc')
| -rw-r--r-- | absl/base/internal/spinlock.cc | 243 |
1 files changed, 243 insertions, 0 deletions
diff --git a/absl/base/internal/spinlock.cc b/absl/base/internal/spinlock.cc new file mode 100644 index 0000000..6257bfc --- /dev/null +++ b/absl/base/internal/spinlock.cc @@ -0,0 +1,243 @@ +// Copyright 2017 The Abseil Authors. +// +// Licensed under the Apache License, Version 2.0 (the "License"); +// you may not use this file except in compliance with the License. +// You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +#include "absl/base/internal/spinlock.h" + +#include <algorithm> +#include <atomic> + +#include "absl/base/casts.h" +#include "absl/base/internal/atomic_hook.h" +#include "absl/base/internal/cycleclock.h" +#include "absl/base/internal/spinlock_wait.h" +#include "absl/base/internal/sysinfo.h" /* For NumCPUs() */ + +// Description of lock-word: +// 31..00: [............................3][2][1][0] +// +// [0]: kSpinLockHeld +// [1]: kSpinLockCooperative +// [2]: kSpinLockDisabledScheduling +// [31..3]: ONLY kSpinLockSleeper OR +// Wait time in cycles >> PROFILE_TIMESTAMP_SHIFT +// +// Detailed descriptions: +// +// Bit [0]: The lock is considered held iff kSpinLockHeld is set. +// +// Bit [1]: Eligible waiters (e.g. Fibers) may co-operatively reschedule when +// contended iff kSpinLockCooperative is set. +// +// Bit [2]: This bit is exclusive from bit [1]. It is used only by a +// non-cooperative lock. When set, indicates that scheduling was +// successfully disabled when the lock was acquired. May be unset, +// even if non-cooperative, if a ThreadIdentity did not yet exist at +// time of acquisition. +// +// Bit [3]: If this is the only upper bit ([31..3]) set then this lock was +// acquired without contention, however, at least one waiter exists. +// +// Otherwise, bits [31..3] represent the time spent by the current lock +// holder to acquire the lock. There may be outstanding waiter(s). + +namespace absl { +namespace base_internal { + +static int adaptive_spin_count = 0; + +namespace { +struct SpinLock_InitHelper { + SpinLock_InitHelper() { + // On multi-cpu machines, spin for longer before yielding + // the processor or sleeping. Reduces idle time significantly. + if (base_internal::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; + +ABSL_CONST_INIT static base_internal::AtomicHook<void (*)(const void *lock, + int64_t wait_cycles)> + submit_profile_data; + +} // namespace + +void RegisterSpinLockProfiler(void (*fn)(const void *contendedlock, + int64_t wait_cycles)) { + submit_profile_data.Store(fn); +} + +static inline bool IsCooperative( + base_internal::SchedulingMode scheduling_mode) { + return scheduling_mode == base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL; +} + +// Uncommon constructors. +SpinLock::SpinLock(base_internal::SchedulingMode mode) + : lockword_(IsCooperative(mode) ? kSpinLockCooperative : 0) { + ABSL_TSAN_MUTEX_CREATE(this, 0); +} + +SpinLock::SpinLock(base_internal::LinkerInitialized, + base_internal::SchedulingMode mode) { + ABSL_TSAN_MUTEX_CREATE(this, __tsan_mutex_linker_init); + if (IsCooperative(mode)) { + InitLinkerInitializedAndCooperative(); + } + // Otherwise, lockword_ is already initialized. +} + +// Static (linker initialized) spinlocks always start life as functional +// non-cooperative locks. When their static constructor does run, it will call +// this initializer to augment the lockword with the cooperative bit. By +// actually taking the lock when we do this we avoid the need for an atomic +// operation in the regular unlock path. +// +// SlowLock() must be careful to re-test for this bit so that any outstanding +// waiters may be upgraded to cooperative status. +void SpinLock::InitLinkerInitializedAndCooperative() { + Lock(); + lockword_.fetch_or(kSpinLockCooperative, std::memory_order_relaxed); + Unlock(); +} + +// 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. +uint32_t SpinLock::SpinLoop(int64_t initial_wait_timestamp, + uint32_t *wait_cycles) { + int c = adaptive_spin_count; + uint32_t lock_value; + do { + lock_value = lockword_.load(std::memory_order_relaxed); + } while ((lock_value & kSpinLockHeld) != 0 && --c > 0); + uint32_t spin_loop_wait_cycles = + EncodeWaitCycles(initial_wait_timestamp, CycleClock::Now()); + *wait_cycles = spin_loop_wait_cycles; + + return TryLockInternal(lock_value, spin_loop_wait_cycles); +} + +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_t wait_start_time = CycleClock::Now(); + uint32_t wait_cycles; + uint32_t lock_value = SpinLoop(wait_start_time, &wait_cycles); + + int lock_wait_call_count = 0; + while ((lock_value & kSpinLockHeld) != 0) { + // If the lock is currently held, but not marked as having a sleeper, mark + // it as having a sleeper. + if ((lock_value & kWaitTimeMask) == 0) { + // 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. + if (lockword_.compare_exchange_strong( + lock_value, lock_value | kSpinLockSleeper, + std::memory_order_acquire, std::memory_order_relaxed)) { + // 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 & kSpinLockHeld) == 0) { + // Lock is free again, so try and acquire it before sleeping. The + // new lock state will be the number of cycles this thread waited if + // this thread obtains the lock. + lock_value = TryLockInternal(lock_value, wait_cycles); + continue; // Skip the delay at the end of the loop. + } + } + + base_internal::SchedulingMode scheduling_mode; + if ((lock_value & kSpinLockCooperative) != 0) { + scheduling_mode = base_internal::SCHEDULE_COOPERATIVE_AND_KERNEL; + } else { + scheduling_mode = base_internal::SCHEDULE_KERNEL_ONLY; + } + // SpinLockDelay() calls into fiber scheduler, we need to see + // synchronization there to avoid false positives. + ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0); + // Wait for an OS specific delay. + base_internal::SpinLockDelay(&lockword_, lock_value, ++lock_wait_call_count, + scheduling_mode); + ABSL_TSAN_MUTEX_POST_DIVERT(this, 0); + // 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); + } +} + +void SpinLock::SlowUnlock(uint32_t lock_value) { + base_internal::SpinLockWake(&lockword_, + false); // wake waiter if necessary + + // If our acquisition was contended, collect contentionz profile info. We + // reserve a unitary wait time to represent that a waiter exists without our + // own acquisition having been contended. + if ((lock_value & kWaitTimeMask) != kSpinLockSleeper) { + const uint64_t wait_cycles = DecodeWaitCycles(lock_value); + ABSL_TSAN_MUTEX_PRE_DIVERT(this, 0); + submit_profile_data(this, wait_cycles); + ABSL_TSAN_MUTEX_POST_DIVERT(this, 0); + } +} + +// We use the upper 29 bits of the lock word to store the time spent waiting to +// acquire this lock. This is reported by contentionz profiling. Since the +// lower bits of the cycle counter wrap very quickly on high-frequency +// processors we divide to reduce the granularity to 2^PROFILE_TIMESTAMP_SHIFT +// sized units. On a 4Ghz machine this will lose track of wait times greater +// than (2^29/4 Ghz)*128 =~ 17.2 seconds. Such waits should be extremely rare. +enum { PROFILE_TIMESTAMP_SHIFT = 7 }; +enum { LOCKWORD_RESERVED_SHIFT = 3 }; // We currently reserve the lower 3 bits. + +uint32_t SpinLock::EncodeWaitCycles(int64_t wait_start_time, + int64_t wait_end_time) { + static const int64_t kMaxWaitTime = + std::numeric_limits<uint32_t>::max() >> LOCKWORD_RESERVED_SHIFT; + int64_t scaled_wait_time = + (wait_end_time - wait_start_time) >> PROFILE_TIMESTAMP_SHIFT; + + // Return a representation of the time spent waiting that can be stored in + // the lock word's upper bits. bit_cast is required as Atomic32 is signed. + const uint32_t clamped = static_cast<uint32_t>( + std::min(scaled_wait_time, kMaxWaitTime) << LOCKWORD_RESERVED_SHIFT); + + // bump up value if necessary to avoid returning kSpinLockSleeper. + const uint32_t after_spinlock_sleeper = + kSpinLockSleeper + (1 << LOCKWORD_RESERVED_SHIFT); + return clamped == kSpinLockSleeper ? after_spinlock_sleeper : clamped; +} + +uint64_t SpinLock::DecodeWaitCycles(uint32_t lock_value) { + // Cast to uint32_t first to ensure bits [63:32] are cleared. + const uint64_t scaled_wait_time = + static_cast<uint32_t>(lock_value & kWaitTimeMask); + return scaled_wait_time + << (PROFILE_TIMESTAMP_SHIFT - LOCKWORD_RESERVED_SHIFT); +} + +} // namespace base_internal +} // namespace absl |