linux-kernel - Re: [RFC PATCHSET] workqueue: implement concurrency managed workqueue

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Message-ID: <4AC4E195.8020307@redhat.com>
Date:	Thu, 01 Oct 2009 19:06:29 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	jeff@...zik.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com
Subject: Re: [RFC PATCHSET] workqueue: implement concurrency managed	workqueue

On 10/01/2009 06:55 PM, Tejun Heo wrote:
>> For kvm, we don't want to force a specific scheduling class for vcpu
>> threads, so we'd need infrastructure to create a new scheduling class
>> out of an existing one to hook the two callbacks.  Seems like quite a
>> lot of work, for something that is orthogonal to scheduling.
>>
>> Tejun, would preempt notifiers work for your workqueues?  see bottom of
>> include/linux/preempt.h.
>>      
> I considered that but the thing is workqueue needs to know when a
> thread wakes up not when it gets scheduled.  Of course we can add
> another notifier op and call it from try_to_wake_up() but I really
> didn't want to add yet another hook in a very hot path which will only
> be useful for very small number of tasks but yet has to be called for
> every operation and the sched_class mechanism means that we already
> have hooks at all the interesting spots, so I think it's better to
> make use of them instead of adding another set of callbacks.
>    

Right, it's a subtle difference that makes p_n unusable for you.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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