eventpoll: Set epoll timeout if it's in the future

eventpoll: Set epoll timeout if it's in the future

Avoid an edge case where epoll_wait arms a timer and calls schedule()
even if the timer will expire immediately.

For example: if the user has specified an epoll busy poll usecs which is
equal or larger than the epoll_wait/epoll_pwait2 timeout, it is
unnecessary to call schedule_hrtimeout_range; the busy poll usecs have
consumed the entire timeout duration so it is unnecessary to induce
scheduling latency by calling schedule() (via schedule_hrtimeout_range).

This can be measured using a simple bpftrace script:

tracepoint:sched:sched_switch
/ args->prev_pid == $1 /
{
  print(kstack());
  print(ustack());
}

Before this patch is applied:

  Testing an epoll_wait app with busy poll usecs set to 1000, and
  epoll_wait timeout set to 1ms using the script above shows:

     __traceiter_sched_switch+69
     __schedule+1495
     schedule+32
     schedule_hrtimeout_range+159
     do_epoll_wait+1424
     __x64_sys_epoll_wait+97
     do_syscall_64+95
     entry_SYSCALL_64_after_hwframe+118

     epoll_wait+82

  Which is unexpected; the busy poll usecs should have consumed the
  entire timeout and there should be no reason to arm a timer.

After this patch is applied: the same test scenario does not generate a
call to schedule() in the above edge case. If the busy poll usecs are
reduced (for example usecs: 100, epoll_wait timeout 1ms) the timer is
armed as expected.

Fixes: bf3b9f6372c4 ("epoll: Add busy poll support to epoll with socket fds.")
Signed-off-by: Joe Damato <jdamato@fastly.com>
Link: https://lore.kernel.org/20250416185826.26375-1-jdamato@fastly.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>