Use sched_yield() as fallback for preemption yield

[triplef]

pthread_yield() is not available on Android.

In combination with #458 this fixes all build issues using Android NDK targeting API level 21 and up.

[compnerd]

This wont work. #453 fix the Windows path to use Sleep(0);

[triplef]

I see. I’ll wait for #453 to be merged and re-do the this to also use sleep on Android.

[triplef]

I’ve updated the patch to use sleep(0) if pthread_yield_np() is not available.

[adierking]

Using sleep(0) to yield on non-Windows systems isn't correct because there's no guarantee that it will actually do anything (and in fact some versions of glibc will return immediately). The Windows path uses it because Microsoft explicitly documents that Sleep(0) relinquishes the thread's timeslice. The correct fallback should be sched_yield(), which is mandated by POSIX.

[triplef]

@adierking that’s what I thought and had used originally. :-) Just to confirm, would you use sched_yield() just on Android or everywhere as a fallback?

[adierking]

@adierking that’s what I thought and had used originally. :-) Just to confirm, would you use sched_yield() just on Android or everywhere as a fallback?

I think it should be used as a fallback everywhere. Nothing should be using pthread_yield() anymore because sched_yield() is a standardized function which accomplishes the same thing.

[triplef]

Great, I updated the patch to use sched_yield().

[compnerd]

@swift-ci please test

[ktopley-apple]

@swift-ci please test

[compnerd]

@swift-ci please test