runtime: use eventfd as the event wait/notify mechanism for epoll

[panjf2000]

Currently, we use pipe to interrupt the epoll_wait, which requires two file descriptors. Furthermore, given its complexity inside, pipe is a bit heavyweight for just a simple event wait/notify mechanism.

eventfd would be a more suitable solution for the current netpoll with epoll in Go, kernel also advocates for developers to use eventfd instead of pipe in some simple scenarios:

Applications can use an eventfd file descriptor instead of a pipe
(see pipe(2)) in all cases where a pipe is used simply to signal
events. The kernel overhead of an eventfd file descriptor is
much lower than that of a pipe, and only one file descriptor is
required (versus the two required for a pipe).

eventfd had been available since Linux 2.6.22 and eventfd2 had been available since Linux 2.6.27, the minimal kernel version required by Go is 2.6.32, so we're good to go.

CC @golang/runtime

[gopherbot]

Change https://go.dev/cl/560615 mentions this issue: runtime: use eventfd as the event wait/notify mechanism for epoll