1 // -*- C++ -*- 2 //===----------------------------------------------------------------------===// 3 // 4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 5 // See https://llvm.org/LICENSE.txt for license information. 6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 7 // 8 //===----------------------------------------------------------------------===// 9 10 #ifndef _LIBCPP___THREAD_POLL_WITH_BACKOFF_H 11 #define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H 12 13 #include <__availability> 14 #include <__chrono/duration.h> 15 #include <__chrono/high_resolution_clock.h> 16 #include <__config> 17 18 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 19 # pragma GCC system_header 20 #endif 21 22 _LIBCPP_BEGIN_NAMESPACE_STD 23 24 static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64; 25 26 // Polls a thread for a condition given by a predicate, and backs off based on a backoff policy 27 // before polling again. 28 // 29 // - __f is the "test function" that should return true if polling succeeded, and false if it failed. 30 // 31 // - __bf is the "backoff policy", which is called with the duration since we started polling. It should 32 // return false in order to resume polling, and true if polling should stop entirely for some reason. 33 // In general, backoff policies sleep for some time before returning control to the polling loop. 34 // 35 // - __max_elapsed is the maximum duration to try polling for. If the maximum duration is exceeded, 36 // the polling loop will return false to report a timeout. 37 template<class _Fn, class _BFn> 38 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI 39 bool __libcpp_thread_poll_with_backoff(_Fn&& __f, _BFn&& __bf, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero()) { 40 auto const __start = chrono::high_resolution_clock::now(); 41 for (int __count = 0;;) { 42 if (__f()) 43 return true; // _Fn completion means success 44 if (__count < __libcpp_polling_count) { 45 __count += 1; 46 continue; 47 } 48 chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start; 49 if (__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed) 50 return false; // timeout failure 51 if (__bf(__elapsed)) 52 return false; // _BFn completion means failure 53 } 54 } 55 56 // A trivial backoff policy that always immediately returns the control to 57 // the polling loop. 58 // 59 // This is not very well-behaved since it will cause the polling loop to spin, 60 // so this should most likely only be used on single-threaded systems where there 61 // are no other threads to compete with. 62 struct __spinning_backoff_policy { 63 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR 64 bool operator()(chrono::nanoseconds const&) const { 65 return false; 66 } 67 }; 68 69 _LIBCPP_END_NAMESPACE_STD 70 71 #endif // _LIBCPP___THREAD_POLL_WITH_BACKOFF_H 72