From 0a65bc27bd645894175c059397b4916e31955fb2 Mon Sep 17 00:00:00 2001 From: Joe Damato Date: Wed, 16 Apr 2025 18:58:25 +0000 Subject: [PATCH] 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 Link: https://lore.kernel.org/20250416185826.26375-1-jdamato@fastly.com Reviewed-by: Jan Kara Signed-off-by: Christian Brauner --- fs/eventpoll.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 100376863a44..4bc264b854c4 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1996,6 +1996,14 @@ static int ep_try_send_events(struct eventpoll *ep, 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. @@ -2103,7 +2111,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, 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);