linux-kernel - Re: [PATCH 4/4] workqueue: Handle ordered workqueues on cpumask

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Message-ID: <20140501145145.GC25369@localhost.localdomain>
Date:	Thu, 1 May 2014 16:51:47 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Kevin Hilman <khilman@...aro.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Mike Galbraith <bitbucket@...ine.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 4/4] workqueue: Handle ordered workqueues on
 cpumask_unbounds change

On Thu, Apr 24, 2014 at 11:33:20AM -0400, Tejun Heo wrote:
> On Thu, Apr 24, 2014 at 04:37:36PM +0200, Frederic Weisbecker wrote:
> > Ordered unbound workqueues need some special care if we want to
> > modify their CPU affinity. These can't be simply handled through
> > apply_workqueue_attrs() since it works by hot plugging worker pools
> > which has parallelism side effects and this would break ordering.
> > 
> > The way we solve this is to change the affinity of the (presumaly
> > unique) worker backing the ordered workqueues.
> > 
> > NOTE: Now like Lai said, there may be bad side effects on this because
> > ordered wq may share their worker pool with non-ordered workqueues.
> > So changing the affinity of the worker itself is not a nice solution.
> > This patch is very likely to be replaced by Lai's patch
> > "workqueue: allow changing attributions of ordered workqueue"
> > https://lkml.org/lkml/2014/4/15/181
> 
> Yeah, it bothers me that we're taking two completely different
> approaches for ordered and !ordered workqueues.  The only difference
> between them is concurrency and it probably would be a better idea to
> address that directly.

Yeah I've tried with Lai's patch and it seems to work like a charm so
next version will likely be better.
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