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86 lines
3.1 KiB
C++
86 lines
3.1 KiB
C++
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// Copyright 2015 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "base/allocator/partition_allocator/spin_lock.h"
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#if defined(OS_WIN)
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#include <windows.h>
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#elif defined(OS_POSIX)
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#include <sched.h>
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#endif
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// The YIELD_PROCESSOR macro wraps an architecture specific-instruction that
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// informs the processor we're in a busy wait, so it can handle the branch more
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// intelligently and e.g. reduce power to our core or give more resources to the
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// other hyper-thread on this core. See the following for context:
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// https://software.intel.com/en-us/articles/benefitting-power-and-performance-sleep-loops
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//
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// The YIELD_THREAD macro tells the OS to relinquish our quantum. This is
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// basically a worst-case fallback, and if you're hitting it with any frequency
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// you really should be using a proper lock (such as |base::Lock|)rather than
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// these spinlocks.
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#if defined(OS_WIN)
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#define YIELD_PROCESSOR YieldProcessor()
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#define YIELD_THREAD SwitchToThread()
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#elif defined(COMPILER_GCC) || defined(__clang__)
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#if defined(ARCH_CPU_X86_64) || defined(ARCH_CPU_X86)
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#define YIELD_PROCESSOR __asm__ __volatile__("pause")
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#elif (defined(ARCH_CPU_ARMEL) && __ARM_ARCH >= 6) || defined(ARCH_CPU_ARM64)
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#define YIELD_PROCESSOR __asm__ __volatile__("yield")
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#elif defined(ARCH_CPU_MIPSEL)
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// The MIPS32 docs state that the PAUSE instruction is a no-op on older
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// architectures (first added in MIPS32r2). To avoid assembler errors when
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// targeting pre-r2, we must encode the instruction manually.
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#define YIELD_PROCESSOR __asm__ __volatile__(".word 0x00000140")
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#elif defined(ARCH_CPU_MIPS64EL) && __mips_isa_rev >= 2
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// Don't bother doing using .word here since r2 is the lowest supported mips64
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// that Chromium supports.
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#define YIELD_PROCESSOR __asm__ __volatile__("pause")
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#endif
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#endif
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#ifndef YIELD_PROCESSOR
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#warning "Processor yield not supported on this architecture."
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#define YIELD_PROCESSOR ((void)0)
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#endif
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#ifndef YIELD_THREAD
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#if defined(OS_POSIX)
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#define YIELD_THREAD sched_yield()
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#else
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#warning "Thread yield not supported on this OS."
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#define YIELD_THREAD ((void)0)
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#endif
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#endif
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namespace base {
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namespace subtle {
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SpinLock::SpinLock() = default;
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SpinLock::~SpinLock() = default;
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void SpinLock::LockSlow() {
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// The value of |kYieldProcessorTries| is cargo culted from TCMalloc, Windows
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// critical section defaults, and various other recommendations.
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// TODO(jschuh): Further tuning may be warranted.
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static const int kYieldProcessorTries = 1000;
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do {
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do {
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for (int count = 0; count < kYieldProcessorTries; ++count) {
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// Let the processor know we're spinning.
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YIELD_PROCESSOR;
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if (!lock_.load(std::memory_order_relaxed) &&
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LIKELY(!lock_.exchange(true, std::memory_order_acquire)))
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return;
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}
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// Give the OS a chance to schedule something on this core.
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YIELD_THREAD;
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} while (lock_.load(std::memory_order_relaxed));
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} while (UNLIKELY(lock_.exchange(true, std::memory_order_acquire)));
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}
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} // namespace subtle
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} // namespace base
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