https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113850
--- Comment #5 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-13 branch has been updated by Jonathan Yong <jy...@gcc.gnu.org>: https://gcc.gnu.org/g:b5def8fd08976fc2514426ecb1f263f13ba3e2af commit r13-8337-gb5def8fd08976fc2514426ecb1f263f13ba3e2af Author: Matteo Italia <mat...@mitalia.net> Date: Fri Feb 9 15:04:20 2024 +0100 libgcc: fix Win32 CV abnormal spurious wakeups in timed wait [PR113850] Fix a typo in __gthr_win32_abs_to_rel_time that caused it to return a relative time in seconds instead of milliseconds. As a consequence, __gthr_win32_cond_timedwait called SleepConditionVariableCS with a 1000x shorter timeout; this caused ~1000x more spurious wakeups in CV timed waits such as std::condition_variable::wait_for or wait_until, resulting generally in much higher CPU usage. This can be demonstrated by this sample program: ``` int main() { std::condition_variable cv; std::mutex mx; bool pass = false; auto thread_fn = [&](bool timed) { int wakeups = 0; using sc = std::chrono::system_clock; auto before = sc::now(); std::unique_lock<std::mutex> ml(mx); if (timed) { cv.wait_for(ml, std::chrono::seconds(2), [&]{ ++wakeups; return pass; }); } else { cv.wait(ml, [&]{ ++wakeups; return pass; }); } printf("pass: %d; wakeups: %d; elapsed: %d ms\n", pass, wakeups, int((sc::now() - before) / std::chrono::milliseconds(1))); pass = false; }; { // timed wait, let expire std::thread t(thread_fn, true); t.join(); } { // timed wait, wake up explicitly after 1 second std::thread t(thread_fn, true); std::this_thread::sleep_for(std::chrono::seconds(1)); { std::unique_lock<std::mutex> ml(mx); pass = true; } cv.notify_all(); t.join(); } { // non-timed wait, wake up explicitly after 1 second std::thread t(thread_fn, false); std::this_thread::sleep_for(std::chrono::seconds(1)); { std::unique_lock<std::mutex> ml(mx); pass = true; } cv.notify_all(); t.join(); } return 0; } ``` On builds based on non-affected threading models (e.g. POSIX on Linux, or winpthreads or MCF on Win32) the output is something like ``` pass: 0; wakeups: 2; elapsed: 2000 ms pass: 1; wakeups: 2; elapsed: 991 ms pass: 1; wakeups: 2; elapsed: 996 ms ``` while with the Win32 threading model we get ``` pass: 0; wakeups: 1418; elapsed: 2000 ms pass: 1; wakeups: 479; elapsed: 988 ms pass: 1; wakeups: 2; elapsed: 992 ms ``` (notice the huge number of wakeups in the timed wait cases only). This commit fixes the conversion, adjusting the final division by NSEC100_PER_SEC to use NSEC100_PER_MSEC instead (already defined in the file and not used in any other place, so probably just a typo). libgcc/ChangeLog: PR libgcc/113850 * config/i386/gthr-win32-cond.c (__gthr_win32_abs_to_rel_time): fix absolute timespec to relative milliseconds count conversion (it incorrectly returned seconds instead of milliseconds); this avoids spurious wakeups in __gthr_win32_cond_timedwait (cherry picked from commit 05ad8fb55a55f1e201fd781c84682a7c0bbd4d97)
