Date:   Mon, 7 Oct 2019 12:42:41 -0400
From:   Jason Baron <jbaron@...mai.com>
To:     Roman Penyaev <rpenyaev@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, hev <r@....cc>,
        linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Davide Libenzi <davidel@...ilserver.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Eric Wong <e@...24.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v4] fs/epoll: Remove unnecessary wakeups of nested
 epoll that in ET mode



On 10/7/19 6:54 AM, Roman Penyaev wrote:
> On 2019-10-03 18:13, Jason Baron wrote:
>> On 9/30/19 7:55 AM, Roman Penyaev wrote:
>>> On 2019-09-28 04:29, Andrew Morton wrote:
>>>> On Wed, 25 Sep 2019 09:56:03 +0800 hev <r@....cc> wrote:
>>>>
>>>>> From: Heiher <r@....cc>
>>>>>
>>>>> Take the case where we have:
>>>>>
>>>>>         t0
>>>>>          | (ew)
>>>>>         e0
>>>>>          | (et)
>>>>>         e1
>>>>>          | (lt)
>>>>>         s0
>>>>>
>>>>> t0: thread 0
>>>>> e0: epoll fd 0
>>>>> e1: epoll fd 1
>>>>> s0: socket fd 0
>>>>> ew: epoll_wait
>>>>> et: edge-trigger
>>>>> lt: level-trigger
>>>>>
>>>>> We only need to wakeup nested epoll fds if something has been queued
>>>>> to the
>>>>> overflow list, since the ep_poll() traverses the rdllist during
>>>>> recursive poll
>>>>> and thus events on the overflow list may not be visible yet.
>>>>>
>>>>> Test code:
>>>>
>>>> Look sane to me.  Do you have any performance testing results which
>>>> show a benefit?
>>>>
>>>> epoll maintainership isn't exactly a hive of activity nowadays :(
>>>> Roman, would you please have time to review this?
>>>
>>> So here is my observation: current patch does not fix the described
>>> problem (double wakeup) for the case, when new event comes exactly
>>> to the ->ovflist.  According to the patch this is the desired intention:
>>>
>>>    /*
>>>     * We only need to wakeup nested epoll fds if something has been
>>> queued
>>>     * to the overflow list, since the ep_poll() traverses the rdllist
>>>     * during recursive poll and thus events on the overflow list may
>>> not be
>>>     * visible yet.
>>>     */
>>>     if (nepi != NULL)
>>>        pwake++;
>>>
>>>     ....
>>>
>>>     if (pwake == 2)
>>>        ep_poll_safewake(&ep->poll_wait);
>>>
>>>
>>> but this actually means that we repeat the same behavior (double wakeup)
>>> but only for the case, when event comes to the ->ovflist.
>>>
>>> How to reproduce? Can be easily done (ok, not so easy but it is possible
>>> to try): to the given userspace test we need to add one more socket and
>>> immediately fire the event:
>>>
>>>     e.events = EPOLLIN;
>>>     if (epoll_ctl(efd[1], EPOLL_CTL_ADD, s2fd[0], &e) < 0)
>>>        goto out;
>>>
>>>     /*
>>>      * Signal any fd to let epoll_wait() to call ep_scan_ready_list()
>>>      * in order to "catch" it there and add new event to ->ovflist.
>>>      */
>>>      if (write(s2fd[1], "w", 1) != 1)
>>>         goto out;
>>>
>>> That is done in order to let the following epoll_wait() call to invoke
>>> ep_scan_ready_list(), where we can "catch" and insert new event exactly
>>> to the ->ovflist. In order to insert event exactly in the correct list
>>> I introduce artificial delay.
>>>
>>> Modified test and kernel patch is below.  Here is the output of the
>>> testing tool with some debug lines from kernel:
>>>
>>>   # ~/devel/test/edge-bug
>>>   [   59.263178] ### sleep 2
>>>   >> write to sock
>>>   [   61.318243] ### done sleep
>>>   [   61.318991] !!!!!!!!!!! ep_poll_safewake(&ep->poll_wait);
>>> events_in_rdllist=1, events_in_ovflist=1
>>>   [   61.321204] ### sleep 2
>>>   [   63.398325] ### done sleep
>>>   error: What?! Again?!
>>>
>>> First epoll_wait() call (ep_scan_ready_list()) observes 2 events
>>> (see "!!!!!!!!!!! ep_poll_safewake" output line), exactly what we
>>> wanted to achieve, so eventually ep_poll_safewake() is called again
>>> which leads to double wakeup.
>>>
>>> In my opinion current patch as it is should be dropped, it does not
>>> fix the described problem but just hides it.
>>>
>>> -- 
> 
> Hi Jason,
> 
>>
>> Yes, there are 2 wakeups in the test case you describe, but if the
>> second event (write to s1fd) gets queued after the first call to
>> epoll_wait(), we are going to get 2 wakeups anyways.
> 
> Yes, exactly, for this reason I print out the number of events observed
> on first wait, there should be 1 (rdllist) and 1 (ovflist), otherwise
> this is another case, when second event comes exactly after first
> wait, which is legitimate wakeup.
> 
>> So yes, there may
>> be a slightly bigger window with this patch for 2 wakeups, but its small
>> and I tried to be conservative with the patch - I'd rather get an
>> occasional 2nd wakeup then miss one. Trying to debug missing wakeups
>> isn't always fun...
>>
>> That said, the reason for propagating events that end up on the overflow
>> list was to prevent the race of the wakee not seeing events because they
>> were still on the overflow list. In the testcase, imagine if there was a
>> thread doing epoll_wait() on efd[0], and then a write happends on s1fd.
>> I thought it was possible then that a 2nd thread doing epoll_wait() on
>> efd[1], wakes up, checks efd[0] and sees no events because they are
>> still potentially on the overflow list. However, I think that case is
>> not possible because the thread doing epoll_wait() on efd[0] is going to
>> have the ep->mtx, and thus when the thread wakes up on efd[1], its going
>> to have to be ordered because its also grabbing the ep->mtx associated
>> with efd[0].
>>
>> So I think its safe to do the following if we want to go further than
>> the proposed patch, which is what you suggested earlier in the thread
>> (minus keeping the wakeup on ep->wq).
> 
> Then I do not understand why we need to keep ep->wq wakeup?
> @wq and @poll_wait are almost the same with only one difference:
> @wq is used when you sleep on it inside epoll_wait() and the other
> is used when you nest epoll fd inside epoll fd.  Either you wake
> both up either you don't this at all.
> 
> ep_poll_callback() does wakeup explicitly, ep_insert() and ep_modify()
> do wakeup explicitly, so what are the cases when we need to do wakeups
> from ep_scan_ready_list()?

Hi Roman,

So the reason I was saying not to drop the ep->wq wakeup was that I was
thinking about a usecase where you have multi-threads say thread A and
thread B which are doing epoll_wait() on the same epfd. Now, the threads
both call epoll_wait() and are added as exclusive to ep->wq. Now a bunch
of events happen and thread A is woken up. However, thread A may only
process a subset of the events due to its 'maxevents' parameter. In that
case, I was thinking that the wakeup on ep->wq might be helpful, because
in the absence of subsequent events, thread B can now start processing
the rest, instead of waiting for the next event to be queued.

However, I was thinking about the state of things before:
86c0517 fs/epoll: deal with wait_queue only once

Before that patch, thread A would have been removed from eq->wq before
the wakeup call, thus waking up thread B. However, now that thread A
stays on the queue during the call to ep_send_events(), I believe the
wakeup would only target thread A, which doesn't help since its already
checking for events. So given the state of things I think you are right
in that its not needed. However, I wonder if not removing from the
ep->wq affects the multi-threaded case I described. Its been around
since 5.0, so probably not, but it would be a more subtle performance
difference.

Thanks,

-Jason




> 
> I would still remove the whole branch:
> 
> 
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -671,7 +671,6 @@ static __poll_t ep_scan_ready_list(struct eventpoll
> *ep,
>                               void *priv, int depth, bool ep_locked)
>  {
>         __poll_t res;
> -       int pwake = 0;
>         struct epitem *epi, *nepi;
>         LIST_HEAD(txlist);
> 
> @@ -738,26 +737,11 @@ static __poll_t ep_scan_ready_list(struct
> eventpoll *ep,
>          */
>         list_splice(&txlist, &ep->rdllist);
>         __pm_relax(ep->ws);
> -
> -       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);
> -
>         return res;
>  }
> 
> -- 
> Roman
> 
> 
> 

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