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

From: Roman Penyaev
Date: Mon Oct 07 2019 - 14:31:07 EST

On 2019-10-07 18:42, Jason Baron wrote:
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@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()?

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.

Now I understand what you mean. You want to prevent "idling" of events,
while thread A is busy with the small portion of events (portion is equal
to maxevents). On next iteration thread A will pick up the rest, no doubts,
but would be nice to give a chance to thread B immediately to deal with the
rest. Ok, makes sense.

But what if to make this wakeup explicit if we have more events to process?
(nothing is tested, just a guess)

@@ -255,6 +255,7 @@ struct ep_pqueue {
struct ep_send_events_data {
int maxevents;
struct epoll_event __user *events;
+ bool have_more;
int res;
};
@@ -1783,14 +1768,17 @@ static __poll_t ep_send_events_proc(struct eventpoll *ep, struct list_head *head
}

static int ep_send_events(struct eventpoll *ep,
- struct epoll_event __user *events, int maxevents)
+ struct epoll_event __user *events, int maxevents,
+ bool *have_more)
{
- struct ep_send_events_data esed;
-
- esed.maxevents = maxevents;
- esed.events = events;
+ struct ep_send_events_data esed = {
+ .maxevents = maxevents,
+ .events = events,
+ };

ep_scan_ready_list(ep, ep_send_events_proc, &esed, 0, false);
+ *have_more = esed.have_more;
+
return esed.res;
}

@@ -1827,7 +1815,7 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
{
int res = 0, eavail, timed_out = 0;
u64 slack = 0;
- bool waiter = false;
+ bool waiter = false, have_more;
wait_queue_entry_t wait;
ktime_t expires, *to = NULL;

@@ -1927,7 +1915,8 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
* more luck.
*/
if (!res && eavail &&
- !(res = ep_send_events(ep, events, maxevents)) && !timed_out)
+ !(res = ep_send_events(ep, events, maxevents, &have_more)) &&
+ !timed_out)
goto fetch_events;

if (waiter) {
@@ -1935,6 +1924,12 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
__remove_wait_queue(&ep->wq, &wait);
spin_unlock_irq(&ep->wq.lock);
}
+ /*
+ * We were not able to process all the events, so immediately
+ * wakeup other waiter.
+ */
+ if (res > 0 && have_more && waitqueue_active(&ep->wq))
+ wake_up(&ep->wq);

return res;
}


PS. So what we decide with the original patch? Remove the whole branch?

--
Roman