linux-kernel - Re: [PATCHSET] workqueue: reimplement CPU hotplug to keep idle workers

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Date:	Fri, 20 Jul 2012 10:02:55 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	tglx@...utronix.de, linux-pm@...r.kernel.org
Subject: Re: [PATCHSET] workqueue: reimplement CPU hotplug to keep idle
 workers

Hey, Peter.

On Fri, Jul 20, 2012 at 05:48:31PM +0200, Peter Zijlstra wrote:
> On Tue, 2012-07-17 at 10:12 -0700, Tejun Heo wrote:
> > While this makes rebinding somewhat more complicated, as it has to be
> > able to rebind idle workers too, it allows overall hotplug path to be
> > much simpler.  
> 
> I really don't see the point of re-binding.. at that point you've well
> and proper violated any per-cpu expectation, so why not complete running
> the works on the disassociated thing and let new works accrue on the
> per-cpu things again?

We've discussed this a couple times now, so the existing reasons were,

* Local affinity is more often used as a form of affinity optimization
  since the beginning.  This, mixed with queue_work() /
  queue_work_on(), does make things muddy.

* With local affinity used for optimization, we better support
  detaching running workers - before cmwq, this used to be one of the
  sources of trouble during power state changes.

* So, we have unbound workers which started as bound while a CPU is
  down.  When the CPU comes back up again, we can do one of the
  followings - 1. migrate the unbound ones to WORK_CPU_UNBOUND (can
  also do this on CPU_DOWN), 2. leave them unbound and keep them
  running in parallel with bound ones, or 3. rebind them.  #2 is the
  hariest - it contaminates the usual !hotplug code paths.  #1 or #3,
  unsure, but given how global_cwq's don't usually interact with each
  other, I thought #3 would be lower impact on hot paths.

So, the above was my rationale before this "we need to stop destroying
and re-creating kthreads across CPU hotplug events because phones do
it gazillion times".  Now, I don't think we have any other way.

Thanks.

-- 
tejun
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