linux-kernel - Re: [RFC] sched: implement the exclusive wait queue as a LIFO queue

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Date:	Wed, 28 Apr 2010 11:57:01 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Changli Gao <xiaosuo@...il.com>
cc:	David Howells <dhowells@...hat.com>,
	Yong Zhang <yong.zhang@...driver.com>,
	Xiaotian Feng <xtfeng@...il.com>, Ingo Molnar <mingo@...e.hu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Roland Dreier <rolandd@...co.com>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	Christoph Lameter <cl@...ux.com>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Takashi Iwai <tiwai@...e.de>, linux-fsdevel@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] sched: implement the exclusive wait queue as a LIFO
 queue

On Wed, 28 Apr 2010, Changli Gao wrote:

> On Wed, Apr 28, 2010 at 5:29 PM, David Howells <dhowells@...hat.com> wrote:
> > Changli Gao <xiaosuo@...il.com> wrote:
> >
> >> If there isn't enough work to be done, we'd better not disrupt them
> >> and  leave them sleeping forever to keep the scheduler happier. Do we
> >> have reason to keep fair to all the workers? Does it have benefit?
> >
> > You've made one important assumption: the processes on the wait queue are
> > sleeping waiting to service things... but what if the wait queue governs
> > access to a resource, and all the processes on that wait queue need access to
> > that resource to do things?  Some of the processes waiting for it may never
> > get a go, and so necessary work may be left undone.
> >
> 
> You are right. I made the wrong assumption. But we indeed need some
> primitive to add wait_queue at the head of the wait_queue_head, and I
> know epoll needs it, at least.
> 
> fs/eventpoll.c: 1443.
>                 wait.flags |= WQ_FLAG_EXCLUSIVE;
>                 __add_wait_queue(&ep->wq, &wait);

I'm not sure one user deserves a new function, but like it has been 
noticed, the patch for that should eventually be totally isolated from 
other bits.


- Davide