Re: [PATCH RESEND] fs/epoll: fix the edge-triggered mode for nested epoll
From: Roman Penyaev
Date: Thu Sep 05 2019 - 13:27:18 EST
On 2019-09-05 11:56, Heiher wrote:
Hi,
On Thu, Sep 5, 2019 at 10:53 AM Heiher <r@xxxxxx> wrote:
Hi,
I created an epoll wakeup test project, listed some possible cases,
and any other corner cases needs to be added?
https://github.com/heiher/epoll-wakeup/blob/master/README.md
On Wed, Sep 4, 2019 at 10:02 PM Heiher <r@xxxxxx> wrote:
>
> Hi,
>
> On Wed, Sep 4, 2019 at 8:02 PM Jason Baron <jbaron@xxxxxxxxxx> wrote:
> >
> >
> >
> > On 9/4/19 5:57 AM, Roman Penyaev wrote:
> > > On 2019-09-03 23:08, Jason Baron wrote:
> > >> On 9/2/19 11:36 AM, Roman Penyaev wrote:
> > >>> Hi,
> > >>>
> > >>> This is indeed a bug. (quick side note: could you please remove efd[1]
> > >>> from your test, because it is not related to the reproduction of a
> > >>> current bug).
> > >>>
> > >>> Your patch lacks a good description, what exactly you've fixed. Let
> > >>> me speak out loud and please correct me if I'm wrong, my understanding
> > >>> of epoll internals has become a bit rusty: when epoll fds are nested
> > >>> an attempt to harvest events (ep_scan_ready_list() call) produces a
> > >>> second (repeated) event from an internal fd up to an external fd:
> > >>>
> > >>> epoll_wait(efd[0], ...):
> > >>> ep_send_events():
> > >>> ep_scan_ready_list(depth=0):
> > >>> ep_send_events_proc():
> > >>> ep_item_poll():
> > >>> ep_scan_ready_list(depth=1):
> > >>> ep_poll_safewake():
> > >>> ep_poll_callback()
> > >>> list_add_tail(&epi, &epi->rdllist);
> > >>> ^^^^^^
> > >>> repeated event
> > >>>
> > >>>
> > >>> In your patch you forbid wakeup for the cases, where depth != 0, i.e.
> > >>> for all nested cases. That seems clear. But what if we can go further
> > >>> and remove the whole chunk, which seems excessive:
> > >>>
> > >>> @@ -885,26 +886,11 @@ static __poll_t ep_scan_ready_list(struct
> > >>> eventpoll *ep,
> > >>>
> > >>> -
> > >>> - if (!list_empty(&ep->rdllist)) {
> > >>> - /*
> > >>> - * Wake up (if active) both the eventpoll wait list and
> > >>> - * the ->poll() wait list (delayed after we release the
> > >>> lock).
> > >>> - */
> > >>> - if (waitqueue_active(&ep->wq))
> > >>> - wake_up(&ep->wq);
> > >>> - if (waitqueue_active(&ep->poll_wait))
> > >>> - pwake++;
> > >>> - }
> > >>> write_unlock_irq(&ep->lock);
> > >>>
> > >>> if (!ep_locked)
> > >>> mutex_unlock(&ep->mtx);
> > >>>
> > >>> - /* We have to call this outside the lock */
> > >>> - if (pwake)
> > >>> - ep_poll_safewake(&ep->poll_wait);
> > >>>
> > >>>
> > >>> I reason like that: by the time we've reached the point of scanning events
> > >>> for readiness all wakeups from ep_poll_callback have been already fired and
> > >>> new events have been already accounted in ready list (ep_poll_callback()
> > >>> calls
> > >>> the same ep_poll_safewake()). Here, frankly, I'm not 100% sure and probably
> > >>> missing some corner cases.
> > >>>
> > >>> Thoughts?
> > >>
> > >> So the: 'wake_up(&ep->wq);' part, I think is about waking up other
> > >> threads that may be in waiting in epoll_wait(). For example, there may
> > >> be multiple threads doing epoll_wait() on the same epoll fd, and the
> > >> logic above seems to say thread 1 may have processed say N events and
> > >> now its going to to go off to work those, so let's wake up thread 2 now
> > >> to handle the next chunk.
> > >
> > > Not quite. Thread which calls ep_scan_ready_list() processes all the
> > > events, and while processing those, removes them one by one from the
> > > ready list. But if event mask is !0 and event belongs to
> > > Level Triggered Mode descriptor (let's say default mode) it tails event
> > > again back to the list (because we are in level mode, so event should
> > > be there). So at the end of this traversing loop ready list is likely
> > > not empty, and if so, wake up again is called for nested epoll fds.
> > > But, those nested epoll fds should get already all the notifications
> > > from the main event callback ep_poll_callback(), regardless any thread
> > > which traverses events.
> > >
> > > I suppose this logic exists for decades, when Davide (the author) was
> > > reshuffling the code here and there.
> > >
> > > But I do not feel confidence to state that this extra wakeup is bogus,
> > > I just have a gut feeling that it looks excessive.
> >
> > Note that I was talking about the wakeup done on ep->wq not ep->poll_wait.
> > The path that I'm concerned about is let's say that there are N events
> > queued on the ready list. A thread that was woken up in epoll_wait may
> > decide to only process say N/2 of then. Then it will call wakeup on ep->wq
> > and this will wakeup another thread to process the remaining N/2. Without
> > the wakeup, the original thread isn't going to process the events until
> > it finishes with the original N/2 and gets back to epoll_wait(). So I'm not
> > sure how important that path is but I wanted to at least note the change
> > here would impact that behavior.
> >
> > Thanks,
> >
> > -Jason
> >
> >
> > >
> > >> So I think removing all that even for the
> > >> depth 0 case is going to change some behavior here. So perhaps, it
> > >> should be removed for all depths except for 0? And if so, it may be
> > >> better to make 2 patches here to separate these changes.
> > >>
> > >> For the nested wakeups, I agree that the extra wakeups seem unnecessary
> > >> and it may make sense to remove them for all depths. I don't think the
> > >> nested epoll semantics are particularly well spelled out, and afaict,
> > >> nested epoll() has behaved this way for quite some time. And the current
> > >> behavior is not bad in the way that a missing wakeup or false negative
> > >> would be.
> > >
> > > That's 100% true! For edge mode extra wake up is not a bug, not optimal
> > > for userspace - yes, but that can't lead to any lost wakeups.
> > >
> > > --
> > > Roman
> > >
>
> I tried to remove the whole chunk of code that Roman said, and it
> seems that there
> are no obvious problems with the two test programs below:
I recall this message, the test case 9/25/26 of epoll-wakeup (on
github) are failed while
the whole chunk are removed.
Apply the original patch, all tests passed.
These are failing on my bare 5.2.0-rc2
TEST bin/epoll31 FAIL
TEST bin/epoll46 FAIL
TEST bin/epoll50 FAIL
TEST bin/epoll32 FAIL
TEST bin/epoll19 FAIL
TEST bin/epoll27 FAIL
TEST bin/epoll42 FAIL
TEST bin/epoll34 FAIL
TEST bin/epoll48 FAIL
TEST bin/epoll40 FAIL
TEST bin/epoll20 FAIL
TEST bin/epoll28 FAIL
TEST bin/epoll38 FAIL
TEST bin/epoll52 FAIL
TEST bin/epoll24 FAIL
TEST bin/epoll23 FAIL
These are failing if your patch is applied:
(my 5.2.0-rc2 is old? broken?)
TEST bin/epoll46 FAIL
TEST bin/epoll42 FAIL
TEST bin/epoll34 FAIL
TEST bin/epoll48 FAIL
TEST bin/epoll40 FAIL
TEST bin/epoll44 FAIL
TEST bin/epoll38 FAIL
These are failing if "ep_poll_safewake(&ep->poll_wait)" is not called,
but wakeup(&ep->wq); is still invoked:
TEST bin/epoll46 FAIL
TEST bin/epoll42 FAIL
TEST bin/epoll34 FAIL
TEST bin/epoll40 FAIL
TEST bin/epoll44 FAIL
TEST bin/epoll38 FAIL
So at least 48 has been "fixed".
These are failing if the whole chunk is removed, like your
said 9,25,26 are among which do not pass:
TEST bin/epoll26 FAIL
TEST bin/epoll42 FAIL
TEST bin/epoll34 FAIL
TEST bin/epoll9 FAIL
TEST bin/epoll48 FAIL
TEST bin/epoll40 FAIL
TEST bin/epoll25 FAIL
TEST bin/epoll44 FAIL
TEST bin/epoll38 FAIL
This can be a good test suite, probably can be added to kselftests?
--
Roman