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 <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Reviewed-by: Jan Kara <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

fs/eventpoll.c

···
 	return res;
 }
 
+static int ep_schedule_timeout(ktime_t *to)
+{
+	if (to)
+		return ktime_after(*to, ktime_get());
+	else
+		return 1;
+}
+
 /**
  * ep_poll - Retrieves ready events, and delivers them to the caller-supplied
  *           event buffer.
···
 
 		write_unlock_irq(&ep->lock);
 
-		if (!eavail)
+		if (!eavail && ep_schedule_timeout(to))
 			timed_out = !schedule_hrtimeout_range(to, slack,
 							      HRTIMER_MODE_ABS);
 		__set_current_state(TASK_RUNNING);