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From: Tejun Heo <tj@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, torvalds@linux-foundation.org,
	tglx@linutronix.de, linux-pm@vger.kernel.org
Subject: Re: [PATCHSET] workqueue: reimplement CPU hotplug to keep idle workers
Date: Fri, 20 Jul 2012 10:02:55 -0700	[thread overview]
Message-ID: <20120720170255.GE32763@google.com> (raw)
In-Reply-To: <1342799311.2583.7.camel@twins>

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

  reply	other threads:[~2012-07-20 17:03 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-07-17 17:12 [PATCHSET] workqueue: reimplement CPU hotplug to keep idle workers Tejun Heo
2012-07-17 17:12 ` [PATCH 1/9] workqueue: perform cpu down operations from low priority cpu_notifier() Tejun Heo
2012-07-20 21:52   ` Paul E. McKenney
2012-07-20 21:58     ` Tejun Heo
2012-07-21 21:36       ` Paul E. McKenney
2012-07-22 16:43         ` [PATCH] workqueue: fix spurious CPU locality WARN from process_one_work() Tejun Heo
2012-07-22 21:23           ` Paul E. McKenney
2012-07-17 17:12 ` [PATCH 2/9] workqueue: drop CPU_DYING notifier operation Tejun Heo
2012-07-17 17:12 ` [PATCH 3/9] workqueue: ROGUE workers are UNBOUND workers Tejun Heo
2012-07-17 17:12 ` [PATCH 4/9] workqueue: use mutex for global_cwq manager exclusion Tejun Heo
2012-07-17 17:12 ` [PATCH 5/9] workqueue: drop @bind from create_worker() Tejun Heo
2012-07-17 17:12 ` [PATCH 6/9] workqueue: reimplement CPU online rebinding to handle idle workers Tejun Heo
2012-07-17 17:12 ` [PATCH 7/9] workqueue: don't butcher idle workers on an offline CPU Tejun Heo
2012-07-17 17:12 ` [PATCH 8/9] workqueue: remove CPU offline trustee Tejun Heo
2012-07-17 17:12 ` [PATCH 9/9] workqueue: simplify CPU hotplug code Tejun Heo
2012-07-17 18:43 ` [PATCHSET] workqueue: reimplement CPU hotplug to keep idle workers Rafael J. Wysocki
2012-07-17 19:40   ` Tejun Heo
2012-07-20 15:48 ` Peter Zijlstra
2012-07-20 17:02   ` Tejun Heo [this message]
2012-07-20 17:21     ` Peter Zijlstra
2012-07-20 17:50       ` Tejun Heo
2012-07-20 18:22         ` Peter Zijlstra
2012-07-20 18:34           ` Tejun Heo
2012-07-20 19:44             ` Rafael J. Wysocki
2012-07-20 19:41               ` Tejun Heo
2012-07-21  6:42               ` Shilimkar, Santosh
2012-07-23  8:38               ` Peter De Schrijver
2012-07-20 16:39 ` Peter Zijlstra
2012-07-20 16:52   ` Tejun Heo
2012-07-20 17:01     ` Peter Zijlstra
2012-07-20 17:08       ` Tejun Heo
2012-07-20 17:19         ` Peter Zijlstra
2012-07-20 17:43           ` Tejun Heo

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