368faacac7
This commit reverts 5aaefa51 (and also partly 7f285f48e77 and b6d75682f9, which were related to the original commit). As landed, 5aaefa51 had unintended consequences on some downstream bots and didn't have proper coverage upstream due to a few subtle things. Implementing this is something we should do in libc++, however we'll first need to address a few issues listed in https://reviews.llvm.org/D106124#3349710. Differential Revision: https://reviews.llvm.org/D120683
74 lines
2.8 KiB
C++
74 lines
2.8 KiB
C++
// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
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#define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
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#include <__availability>
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#include <__chrono/duration.h>
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#include <__chrono/high_resolution_clock.h>
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#include <__chrono/steady_clock.h>
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#include <__chrono/time_point.h>
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#include <__config>
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#include <__filesystem/file_time_type.h>
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#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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#endif
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_LIBCPP_BEGIN_NAMESPACE_STD
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static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64;
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// Polls a thread for a condition given by a predicate, and backs off based on a backoff policy
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// before polling again.
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//
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// - __f is the "test function" that should return true if polling succeeded, and false if it failed.
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//
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// - __bf is the "backoff policy", which is called with the duration since we started polling. It should
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// return false in order to resume polling, and true if polling should stop entirely for some reason.
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// In general, backoff policies sleep for some time before returning control to the polling loop.
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//
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// - __max_elapsed is the maximum duration to try polling for. If the maximum duration is exceeded,
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// the polling loop will return false to report a timeout.
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template<class _Fn, class _BFn>
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_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI
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bool __libcpp_thread_poll_with_backoff(_Fn&& __f, _BFn&& __bf, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero()) {
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auto const __start = chrono::high_resolution_clock::now();
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for (int __count = 0;;) {
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if (__f())
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return true; // _Fn completion means success
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if (__count < __libcpp_polling_count) {
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__count += 1;
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continue;
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}
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chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start;
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if (__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed)
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return false; // timeout failure
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if (__bf(__elapsed))
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return false; // _BFn completion means failure
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}
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}
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// A trivial backoff policy that always immediately returns the control to
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// the polling loop.
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//
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// This is not very well-behaved since it will cause the polling loop to spin,
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// so this should most likely only be used on single-threaded systems where there
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// are no other threads to compete with.
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struct __spinning_backoff_policy {
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_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR
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bool operator()(chrono::nanoseconds const&) const {
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return false;
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}
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};
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_LIBCPP_END_NAMESPACE_STD
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#endif // _LIBCPP___THREAD_POLL_WITH_BACKOFF_H
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