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Message-ID: <CAHirt9i42K37J9n8smaudJyigRAiiDhzZBuW+gbyLXHVq98yqQ@mail.gmail.com>
Date: Tue, 24 Sep 2019 22:06:32 +0800
From: Heiher <r@....cc>
To: Jason Baron <jbaron@...mai.com>
Cc: linux-fsdevel@...r.kernel.org, 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
Hi,
On Mon, Sep 23, 2019 at 11:34 PM Jason Baron <jbaron@...mai.com> wrote:
>
>
>
> 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?
Hmm. I think this is more accurate.
Thank you.
>
>
> 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;
>
--
Best regards!
Hev
https://hev.cc
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