linux-kernel - Re: [PATCH RESEND v2] fs/epoll: Remove unnecessary wakeups of nested epoll that in ET mode

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Date:   Fri, 20 Sep 2019 12:00:09 -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 9/19/19 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;