Re: [PATCH vfs/vfs.fixes v2] eventpoll: Set epoll timeout if it's in the future

[Christian Brauner <brauner@kernel.org>]

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@fastly.com>
> Reviewed-by: Jan Kara <jack@suse.cz>
> ---
>  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@fastly.com/
> 
>  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?