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

[Joe Damato <jdamato@fastly.com>]

On Mon, Apr 28, 2025 at 02:14:45PM +0200, Jan Kara wrote:
> On Sat 26-04-25 14:29:15, Christian Brauner wrote:
> > 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?
> 
> Yep, looks like that. Good spotting!

Thank you for spotting that and sorry for the trouble.

Christian / Jan what would be the correct way for me to deal with
this? Would it be to post a v3 (re-submitting the patch in its
entirety) or to post a new patch that fixes the original and lists
the commit sha from vfs.fixes with a Fixes tag ?

I was going to propose the following, which if I understand
correctly, should fix the issue Christian identified:

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 4bc264b854c4..1a5d1147f082 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -2111,7 +2111,9 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,

                write_unlock_irq(&ep->lock);

-               if (!eavail && ep_schedule_timeout(to))
+               if (!ep_schedule_timeout(to))
+                       timed_out = 1;
+               else if (!eavail)
                        timed_out = !schedule_hrtimeout_range(to, slack,
                                                              HRTIMER_MODE_ABS);
                __set_current_state(TASK_RUNNING);