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Message-ID: <5042e1e0-f49a-74c8-61f8-6903288110ac@akamai.com>
Date: Mon, 23 Sep 2019 11:33:28 -0400
From: Jason Baron <jbaron@...mai.com>
To: hev <r@....cc>, linux-fsdevel@...r.kernel.org
Cc: Al Viro <viro@...IV.linux.org.uk>,
Andrew Morton <akpm@...ux-foundation.org>,
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>,
Roman Penyaev <rpenyaev@...e.de>,
Sridhar Samudrala <sridhar.samudrala@...el.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v2] fs/epoll: Remove unnecessary wakeups of nested
epoll that in ET mode
On 9/20/19 12:00 PM, Jason Baron wrote:
> On 9/19/19 5:24 AM, hev 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
>>
>> When s0 fires an event, e1 catches the event, and then e0 catches an event from
>> e1. After this, There is a thread t0 do epoll_wait() many times on e0, it should
>> only get one event in total, because e1 is a dded to e0 in edge-triggered mode.
>>
>> This patch only allows the wakeup(&ep->poll_wait) in ep_scan_ready_list under
>> two conditions:
>>
>> 1. depth == 0.
>> 2. There have event is added to ep->ovflist during processing.
>>
>> Test code:
>> #include <unistd.h>
>> #include <sys/epoll.h>
>> #include <sys/socket.h>
>>
>> int main(int argc, char *argv[])
>> {
>> int sfd[2];
>> int efd[2];
>> struct epoll_event e;
>>
>> if (socketpair(AF_UNIX, SOCK_STREAM, 0, sfd) < 0)
>> goto out;
>>
>> efd[0] = epoll_create(1);
>> if (efd[0] < 0)
>> goto out;
>>
>> efd[1] = epoll_create(1);
>> if (efd[1] < 0)
>> goto out;
>>
>> e.events = EPOLLIN;
>> if (epoll_ctl(efd[1], EPOLL_CTL_ADD, sfd[0], &e) < 0)
>> goto out;
>>
>> e.events = EPOLLIN | EPOLLET;
>> if (epoll_ctl(efd[0], EPOLL_CTL_ADD, efd[1], &e) < 0)
>> goto out;
>>
>> if (write(sfd[1], "w", 1) != 1)
>> goto out;
>>
>> if (epoll_wait(efd[0], &e, 1, 0) != 1)
>> goto out;
>>
>> if (epoll_wait(efd[0], &e, 1, 0) != 0)
>> goto out;
>>
>> close(efd[0]);
>> close(efd[1]);
>> close(sfd[0]);
>> close(sfd[1]);
>>
>> return 0;
>>
>> out:
>> return -1;
>> }
>>
>> More tests:
>> https://github.com/heiher/epoll-wakeup
>>
>> Cc: Al Viro <viro@...IV.linux.org.uk>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Davide Libenzi <davidel@...ilserver.org>
>> Cc: Davidlohr Bueso <dave@...olabs.net>
>> Cc: Dominik Brodowski <linux@...inikbrodowski.net>
>> Cc: Eric Wong <e@...24.org>
>> Cc: Jason Baron <jbaron@...mai.com>
>> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
>> Cc: Roman Penyaev <rpenyaev@...e.de>
>> Cc: Sridhar Samudrala <sridhar.samudrala@...el.com>
>> Cc: linux-kernel@...r.kernel.org
>> Cc: linux-fsdevel@...r.kernel.org
>> Signed-off-by: hev <r@....cc>
>> ---
>> fs/eventpoll.c | 5 ++++-
>> 1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
>> index c4159bcc05d9..fa71468dbd51 100644
>> --- a/fs/eventpoll.c
>> +++ b/fs/eventpoll.c
>> @@ -685,6 +685,9 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
>> if (!ep_locked)
>> mutex_lock_nested(&ep->mtx, depth);
>>
>> + if (!depth || list_empty_careful(&ep->rdllist))
>> + pwake++;
>> +
>> /*
>> * Steal the ready list, and re-init the original one to the
>> * empty list. Also, set ep->ovflist to NULL so that events
>> @@ -755,7 +758,7 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
>> mutex_unlock(&ep->mtx);
>>
>> /* We have to call this outside the lock */
>> - if (pwake)
>> + if (pwake == 2)
>> ep_poll_safewake(&ep->poll_wait);
>>
>> return res;
>>
>
>
> Hi,
>
> I was thinking more like the following. I tried it using your test-suite
> and it seems to work. What do you think?
>
> Thanks,
>
> -Jason
>
>
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index d7f1f50..662136b 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -712,6 +712,15 @@ static __poll_t ep_scan_ready_list(struct eventpoll
> *ep,
> for (nepi = READ_ONCE(ep->ovflist); (epi = nepi) != NULL;
> nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
> /*
> + * We only need to wakeup nested epoll fds if
> + * 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 (!pwake)
> + pwake++;
> + /*
> * We need to check if the item is already in the list.
> * During the "sproc" callback execution time, items are
> * queued into ->ovflist but the "txlist" might already
> @@ -755,7 +764,7 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
> mutex_unlock(&ep->mtx);
>
> /* We have to call this outside the lock */
> - if (pwake)
> + if (pwake == 2)
> ep_poll_safewake(&ep->poll_wait);
>
> return res;
>
>
Also, probably better to not have that 'if' in the loop, so how about
the following?
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index d7f1f50..ed0d8da 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -704,12 +704,21 @@ static __poll_t ep_scan_ready_list(struct
eventpoll *ep,
res = (*sproc)(ep, &txlist, priv);
write_lock_irq(&ep->lock);
+ nepi = READ_ONCE(ep->ovflist);
+ /*
+ * 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++;
/*
* During the time we spent inside the "sproc" callback, some
* other events might have been queued by the poll callback.
* We re-insert them inside the main ready-list here.
*/
- for (nepi = READ_ONCE(ep->ovflist); (epi = nepi) != NULL;
+ for (; (epi = nepi) != NULL;
nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
/*
* We need to check if the item is already in the list.
@@ -755,7 +764,7 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
mutex_unlock(&ep->mtx);
/* We have to call this outside the lock */
- if (pwake)
+ if (pwake == 2)
ep_poll_safewake(&ep->poll_wait);
return res;
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