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