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

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Date:	Fri, 13 Mar 2015 09:38:27 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	<linux-kernel@...r.kernel.org>, Kevin Hilman <khilman@...aro.org>,
	Mike Galbraith <bitbucket@...ine.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 4/4] workqueue: Allow modifying low level unbound workqueue
 cpumask

On 03/13/2015 01:42 AM, Christoph Lameter wrote:
> On Thu, 12 Mar 2015, Lai Jiangshan wrote:
> 
>> The per-nodes' pwqs are mandatorily controlled by the low level cpumask, while
>> the default pwq ignores the low level cpumask when (and ONLY when) the cpumask set
>> by the user doesn't overlap with the low level cpumask. In this case, we can't
>> apply the empty cpumask to the default pwq, so we use the user-set cpumask
>> directly.
> 
> I am wondering now why we have two cpumasks? 

What's your meaning? which two cpumask?

The per-nodes' pwqs' cpumask and the default pwq's cpumask?
They refer to different pool, so they have different cpumask.

1)
If the per-nodes' pwqs exist, they were controlled by A
(A = user-set-cpumask & possible-cpus-of-the-node). Now after this patch,
they are controlled by B (B = A & the-low-level-cpumak).

if A is empty or B is empty, we used default pwq for the node.

2)
The default pwq is different, it was controlled by C (C = user-set-cpumask),
and after this patch, it will be controlled by D
(D = user-set-cpumask(C) & the-low-level-cpumask), But D may be empty,
we can't have a default pwq with empty cpumask, so we have
use C instead in this case.

> A script can just interate
> through the work queues if we want to set them all right? Then we do not
> have to deal with the conflict between the settings in the kernel.

wq->unbound_attrs->cpumask and the-low-level-cpumask can be set by users.
they may be set different or even non-intersect, the non-intersect case
is not real conflict, but it is possible, we have to handle it.
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