runtime: use epoll_pwait2 where available for sub-millisecond timeout precision on Linux

[ianlancetaylor]

Linux kernel 5.11 (February, 2021) added the epoll_pwait2 system call. It is exactly like epoll_pwait except that the timeout argument is a struct timespec. This permits specifying a timeout with nanosecond precision, where epoll_pwait and epoll_wait only permit millisecond precision. We should change runtime/netpoll_epoll.go to use epoll_pwait2 when available to get higher precision delays.

CC @golang/runtime

[prattmic]

Specifically, this will help with #44343 (comment) on Linux.

[dAdAbird]

Does anybody work on this? I can take on it.

[ianlancetaylor]

Thanks. I suggest that you sketch out what you will do before implementing, to make sure that it seems like a good approach.

[dAdAbird]

@ianlancetaylor, I've just discovered the patch from @prattmic: https://go.dev/cl/363417.
As far as I can tell, it only lacks runtime·epollwait2 implementations for other than amd64 platforms. Are there some reasons you didn't move with the CL 363417 forward? Thanks!

[prattmic]

Mostly because I simply haven't gotten around to it. You are welcome to base a change off of my CL.

I also did want to do some benchmarking (probably with https://cs.opensource.google/go/x/benchmarks/+/master:sweet/) to check for performance regressions, since more precise timeouts could potentially lead to more frequent wakeups.

[dAdAbird]

Thanks, @prattmic. I'll prepare CL then and will do some benchmarking.

[aclements]

Since this is Linux-only, it could use the new runtime/internal/syscall package, rather than implementing it in assembly for all architectures.

[dAdAbird]

Oh, cool, thanks for the advice Austin!