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

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Date:	Fri, 22 Mar 2013 03:24:10 +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 4:52 AM, Eric Wong <normalperson@...t.net> wrote:
> > Changes since v2:
> > * epi->state is no longer atomic, we only cmpxchg in ep_poll_callback
> >   now and rely on implicit barriers in other places for reading.
> > * intermediate EP_STATE_DEQUEUE removed, this (with xchg) caused too
> >   much overhead in the ep_send_events loop and could not eliminate
> >   starvation dangers from improper EPOLLET usage (the original code
> >   had this problem, too, the window is just a few cycles larger, now).
> > * minor code cleanups

> >                 /*
> >                  * Activate ep->ws before deactivating epi->ws to prevent
> 
> Does anything deactivate ep->ws now?

Oops, I left that out when I killed ep_scan_ready_list.
But I think we need a different approach to wakeup sources in
this series...

> > +               /*
> > +                * reset item state for EPOLLONESHOT and EPOLLET
> > +                * no barrier here, rely on ep->mtx release for write barrier
> > +                */
> 
> What happens if ep_poll_callback runs before you set epi->state below?
> It used to queue on ep->ovflist and call __pm_stay_awake on ep->ws,
> but now it does not appear to do anything.
> 
> > +               epi->state = EP_STATE_IDLE;
> >         }
> >
> >         return eventcnt;
> >  }
> >

With EPOLLET and improper usage (not hitting EAGAIN), the event now
has a larger window to be lost (as mentioned in my changelog).

As far as correct __pm_stay_awake/__pm_relax handling, perhaps adding
an atomic counter to struct eventpoll (or each epitem) will work?

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.

Thanks for looking at this patch.

Btw, I'm curious; which applications use EPOLLWAKEUP?

My epoll work is focused on network servers with thousands of clients,
and I don't think any of them use (or have use for) EPOLLWAKEUP.
But I will keep EPOLLWAKEUP users in mind when working on epoll :)
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