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

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Date:	Wed, 28 Apr 2010 19:17:29 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	David Howells <dhowells@...hat.com>
Cc:	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>,
	Davide Libenzi <davidel@...ilserver.org>,
	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@...r.kernel.org
Subject: Re: [RFC] sched: implement the exclusive wait queue as a LIFO queue

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);

-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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