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

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Date:	Sun, 25 Sep 2011 09:02:25 +0300
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Ripduman Sohan <Ripduman.Sohan@...cam.ac.uk>,
	linux-kernel@...r.kernel.org, peterz@...radead.org
Subject: Re: [PATCH] workqueue: Restore cpus_allowed mask for sleeping
 workqueue rescue threads

Hi,

On Sat, Sep 24, 2011 at 6:07 AM, Tejun Heo <tj@...nel.org> wrote:
> On Sun, Sep 18, 2011 at 09:36:57AM +0300, Gilad Ben-Yossef wrote:
>> 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.
>
> Hmm... I don't think this has much to do with CPU isolation.
> Workqueue rescuers are reactively invoked to resolve possible resource
> deadlock.  It doesn't initiate operations on itself while CPU
> isolation requires moving all sources of unwanted activites off the
> isolated CPUs.  Having idle rescuers not bound to any CPU might look
> prettier but wouldn't help anything - it's not an activity source and
> the 'wanted' activities on the isolated CPU also require rescuers for
> forward progress guarantee.
>

Thank you for taking the time to answer my query :-)

I agree there is no real problem with having an additional task bound
to an "isolated"
CPU so long that it does not run and of course that if a task on an
isolated CPU initiated
activity that resulted in requiring the services of a rescuer
workqueue thread it most certainly
needs to run there and that is fine.

I guess my question is - apart form running on the isolated CPU, does
the fact that the
rescuer thread is bound there can cause activity on that CPU
originating from a foreign CPU,
such as for example running an IPI handler in order to migrate it there?

If the answer is negative I agree there is no issue.

You do raise another valid point though - that having the rescuer
workqueue thread bound to an isolated
CPU is "less pretty". Right now it means that someone viewing the set
of tasks in a cpuset, for example,
need to figure out for each task "stuck" on the root set what is it,
why it is bound (and to which CPU) and
if that is a problem. I wonder what we can do to help with that.

Thanks,
Gilad

> Thanks.
>
> --
> tejun
>

-- 
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@...yossef.com
Israel Cell: +972-52-8260388
US Cell: +1-973-8260388
http://benyossef.com

"I've seen things you people wouldn't believe. Goto statements used to
implement co-routines. I watched C structures being stored in
registers. All those moments will be lost in time... like tears in
rain... Time to die. "
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