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

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Message-ID: <4AC4725E.3070509@redhat.com>
Date:	Thu, 01 Oct 2009 11:11:58 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Tejun Heo <tj@...nel.org>, 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 10:40 AM, Ingo Molnar wrote:
> Ok, this looks fairly interesting - and the way you reused scheduler
> classes to auto-regulate with no impact on regular performance is quite
> an ingenious idea as well. (KVM's preempt notifiers should probably use
> this trick too, instead of an ugly notifier in the scheduler hotpath)
>
> This mechanism could be used to implement threadlets/syslets too btw.,
> and other forms of asynchronous IO.
>    

In fact I've thought of implementing threadlets and concurrency-managed 
workqueues with preempt notifiers ;)

Isn't a scheduling class overkill for two existing callbacks?  Note we 
can easily use a thread flag and __switch_to_xtra() to avoid the overhead.

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.

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

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