linux-kernel - Re: [PATCH] workqueue: Restore cpus_allowed mask for sleeping workqueue rescue threads

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Message-ID: <CAOtvUMf0Dx04Piw8bx2oD+F9ttiBP_xCpin7fTvoQqs_dn=ZRQ@mail.gmail.com>
Date:	Sun, 18 Sep 2011 09:36:57 +0300
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Ripduman Sohan <Ripduman.Sohan@...cam.ac.uk>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	peterz@...radead.org
Subject: Re: [PATCH] workqueue: Restore cpus_allowed mask for sleeping
 workqueue rescue threads

Hi

On Thu, Sep 15, 2011 at 7:14 PM, Ripduman Sohan
<Ripduman.Sohan@...cam.ac.uk> wrote:
> Tejun Heo <tj@...nel.org> wrote:
>
>> Hello,
>>
>> On Thu, Sep 01, 2011 at 02:36:33PM +0100, Ripduman Sohan wrote:
>> > Rescuer threads may be migrated (and are bound) to particular CPUs when
>> > active.  However, the allowed_cpus mask is not restored when they return
>> > to sleep rendering inconsistent the presented and actual set of CPUs the
>> > process may potentially run on.  This patch fixes this oversight by
>> > recording the allowed_cpus mask for rescuer threads when it enters the
>> > rescuer_thread() main loop and restoring it every time the thread sleeps.
>>
>> Hmmm... so, currently, rescuer is left bound to the last cpu it worked
>> on.  Why is this a problem?
>

There is at least one use case where I think this can be a problem -
cpu isolation.

If you decide to partition your CPU to give some group of processes a
set of CPUs all to their own (using cgroups/cpuset for example) having
non related bound processes really gets in the way. You would really
want to migrate away non related tasks from the isolated cpuset and
having a bound kernel thread prevents that.

Gilad
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Gilad Ben-Yossef
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