@@ -1980,6 +1980,14 @@ static int ep_autoremove_wake_function(struct wait_queue_entry *wq_entry,
return ret;
}
+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.
@@ -2095,7 +2103,7 @@ 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);
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. Signed-off-by: Joe Damato <jdamato@fastly.com> --- fs/eventpoll.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)