linux-kernel - Re: [RFC v3 1/2] epoll: avoid spinlock contention with wfcqueue

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Message-ID: <20130322103102.GA4818@dcvr.yhbt.net>
Date:	Fri, 22 Mar 2013 10:31:02 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	linux-kernel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC v3 1/2] epoll: avoid spinlock contention with wfcqueue

Arve Hjønnevåg <arve@...roid.com> wrote:
> On Thu, Mar 21, 2013 at 8:24 PM, Eric Wong <normalperson@...t.net> wrote:
> >
> > With EPOLLET and improper usage (not hitting EAGAIN), the event now
> > has a larger window to be lost (as mentioned in my changelog).
> >
> 
> What about the case where EPOLLET is not set? The old code did not
> drop events in that case.

Nothing is dropped, if the event wasn't on the ready list before,
ep_poll_callback may still append the ready list while __put_user
is running.

If the event was on the ready list:

1) It does not matter for EPOLLONESHOT, it'll get masked out and
   discarded in the next ep_send_events call until ep_modify reenables
   it.  Since ep_modify and ep_send_events both take ep->mtx, there's
   no conflict.

2) Level Trigger - event stays ready, so nothing is dropped.

> > As far as correct __pm_stay_awake/__pm_relax handling, perhaps adding
> > an atomic counter to struct eventpoll (or each epitem) will work?
> 
> The wakeup_source should stay in sync with the epoll state. I don't
> think any additional state is needed.

The problem is epi->state is not set atomically in ep_send_events,

Having atomic operations in the loop hurts performance (early versions
of this patch did that, and hurt the single-threaded case).

Maybe I'll only set epi->state atomically if epi->ws is used...

> > If we go with atomic counter in struct eventpoll, is per-epitem
> > wakeup_source still necessary?  We have space in epitem now, but
> > maybe one day we will might need it.
> >
> 
> The wakeup_source per epitem is useful for accounting reasons. If
> suspend fails, it is useful to know which device caused it.

OK.  I'll keep epitem->ws
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