gnu /
gcc /
b5def8fd08976fc2514426ecb1f263f13ba3e2af 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)
1 file changed
