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

[Changli Gao]

On Wed, Apr 28, 2010 at 5:29 PM, David Howells <dhowells@xxxxxxxxxx> wrote:
> Changli Gao <xiaosuo@xxxxxxxxx> wrote:
>
>> If there isn't enough work to be done, we'd better not disrupt them
>> and Âleave 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? ÂSome 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);

-- 
Regardsï
Changli Gao(xiaosuo@xxxxxxxxx)
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