linux-kernel - Re: [PATCH 4/4 V5] workqueue: Allow modifying low level unbound workqueue cpumask

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Date:	Wed, 1 Apr 2015 11:52:12 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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 4/4 V5] workqueue: Allow modifying low level unbound
 workqueue cpumask

On Wed, Apr 01, 2015 at 04:33:30PM +0800, Lai Jiangshan wrote:
> The missing case:
> (@L & @U) is not empty but (@L & @U @O) is empty.
> 
> In my old code (V5 patchset), the dfl_pwq uses (@L & @U), the scheduler will
> use cpu_possible_mask instead due to there is no cpu onlined among all cpu in (@L & @U).
> It is bad, the pwq is NOT controlled by @L nor @U now.
> 
> I think we may use @U for the dfl_pwq in this case.  But it will introduces
> a problem:
> 
> When (@L & @U) has online cpu, the dfl_pwq's cpumaks is (@L & @U).
> when (@L & @U) has no online cpu, the dfl_pwq's cpumask is @U.
> It means dfl_pwq may need to be reallocated during the cpuhotplug-add/remove
> and it means wq_update_unbound_numa() can fail.
> 
> Frederic, TJ, any comments about this case?
> TJ, would you like to make wq_update_unbound_numa() be failure-able?

The goal is making @L behave as cpu_possible_mask for workqueues,
right?  If @L & @U is empty, fallback to @L.  If @L & @O is empty,
leave it to the scheduler.

Thanks.

-- 
tejun
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