On Wed, Apr 16, 2025 at 06:58:25PM +0000, Joe Damato wrote:
> 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@xxxxxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>
> ---
> v2:
> - No longer an RFC and rebased on vfs/vfs.fixes
> - Added Jan's Reviewed-by
> - Added Fixes tag
> - No functional changes from the RFC
>
> rfcv1: https://lore.kernel.org/linux-fsdevel/20250415184346.39229-1-jdamato@xxxxxxxxxx/
>
> 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);
Isn't this buggy? If @to is non-NULL and ep_schedule_timeout() returns
false you want to set timed_out to 1 to break the wait. Otherwise you
hang, no?
