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