Re: [PATCH RESEND v4] fs/epoll: Remove unnecessary wakeups of nested epoll that in ET mode

From: Roman Penyaev
Date: Mon Oct 07 2019 - 06:54:38 EST

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@xxxxxx> wrote:

From: Heiher <r@xxxxxx>

Take the case where we have:

ÂÂÂÂÂÂÂ t0
ÂÂÂÂÂÂÂÂ | (ew)
ÂÂÂÂÂÂÂ e0
ÂÂÂÂÂÂÂÂ | (et)
ÂÂÂÂÂÂÂ e1
ÂÂÂÂÂÂÂÂ | (lt)
ÂÂÂÂÂÂÂ 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:

ÂÂ /*
ÂÂÂ * We only need to wakeup nested epoll fds if something has been queued
ÂÂÂ * to the overflow list, since the ep_poll() traverses the rdllist
ÂÂÂ * during recursive poll and thus events on the overflow list may not be
ÂÂÂ * visible yet.
ÂÂÂ */
ÂÂÂ if (nepi != NULL)
ÂÂÂÂÂÂ pwake++;

ÂÂÂ ....

ÂÂÂ if (pwake == 2)
ÂÂÂÂÂÂ 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:

ÂÂÂ e.events = EPOLLIN;
ÂÂÂ if (epoll_ctl(efd[1], EPOLL_CTL_ADD, s2fd[0], &e) < 0)
ÂÂÂÂÂÂ goto out;

ÂÂÂ /*
ÂÂÂÂ * Signal any fd to let epoll_wait() to call ep_scan_ready_list()
ÂÂÂÂ * in order to "catch" it there and add new event to ->ovflist.
ÂÂÂÂ */
ÂÂÂÂ if (write(s2fd[1], "w", 1) != 1)
ÂÂÂÂÂÂÂ 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()?

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