linux-kernel - Re: [PATCH 3/4] workqueue: Add anon workqueue sysfs hierarchy

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Date:	Thu, 3 Apr 2014 16:42:55 +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>,
	Mike Galbraith <bitbucket@...ine.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 3/4] workqueue: Add anon workqueue sysfs hierarchy

On Sun, Mar 30, 2014 at 09:01:39AM -0400, Tejun Heo wrote:
> On Thu, Mar 27, 2014 at 06:21:01PM +0100, Frederic Weisbecker wrote:
> > We call "anon workqueues" the set of unbound workqueues that don't
> > carry the WQ_SYSFS flag.
> > 
> > They are a problem nowadays because people who work on CPU isolation
> > (HPC, Real time, etc...) want to be able to migrate all the unbound
> > workqueues away to a single housekeeping CPU. This control is possible
> > through sysfs but only with WQ_SYSFS workqueues.
> > 
> > Now we need to deal with the other unbound workqueues. There is two
> > possible solutions:
> > 
> > 1) Implement a sysfs directory for each unbound !WQ_SYSFS. This could
> > be done with a specific Kconfig to make sure that these workqueue
> > won't be considered as a stable ABI. But we all know that all distros
> > will enable this Kconfig symbol and that a warning in the Kconfig help
> > text won't protect against anything.
> > 
> > 2) Implement a single sysfs directory containing only the cpumask file
> > to the control the affinity of all the !WQ_SYSFS workqueues.
> > 
> > This patch implements the second solution but only for non-ordered
> > unbound workqueues. Ordered workqueues need a special treatment and
> > will be handled in a subsequent patch.
> 
> I'm not really sure this is the good approach.  I think I wrote this
> way back but wouldn't it make more sense to allow userland to restrict
> the cpus which are allowed to all unbound cpus.  As currently
> implemented, setting WQ_SYSFS to give userland more control would
> escape that workqueue from this global mask as a side effect, which is
> a surprising behavior and doesn't make much sense to me.

I just considered that anon workqueues shouldn't be that different from
another WQ_SYSFS workqueue. This way we don't have suprising side effect.
Touching a WQ_SYSFS doesn't impact anon workqueues, and touching anon workqueues
doesn't impact WQ_SYSFS workqueues.

In fact this is simply the current way we do it, just extended.

But anyway your solution looks more simple.

> I think it would make far more sense to implement a knob which controls which
> cpus are available to *all* unbound workqueue including the default
> fallback one.  That way it's way more consistent and I'm pretty sure
> the code would be fairly simple too.  All it needs to do is
> restricting the online cpus that unbound workqueues see.

Yeah I like this. So the right place for this cpumask would be in the root of
/sys/devices/virtual/workqueue/ , right?

Thanks.
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