linux-kernel - Re: [PATCH 0/3] resend, cpuset/hotplug fixes

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Date:	Fri, 11 Sep 2009 15:15:14 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Gautham Shenoy <ego@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	Jiri Slaby <jirislaby@...il.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Miao Xie <miaox@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] resend, cpuset/hotplug fixes

Peter Zijlstra wrote:
> On Thu, 2009-09-10 at 21:21 +0200, Oleg Nesterov wrote:
>> (my apologies to those who see this twice).
>>
>>
>> The 1st patch is preparation. 2-3 fix different problems, the 3rd one
>> depends on 2nd.
>>
>> These patches change the code which I don't really understand, please
>> review.
>>
>>
>> As for the 3rd patch, it replaces
>>
>> 	cpu_hotplug-dont-affect-current-tasks-affinity.patch
>>
>> in -mm tree. Imho the new patch is more simple and clean, but of course
>> this is subjective and I am biased.
> 
> Look good to me.
> 
> Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> 
> Ingo, will you stick them in -tip?
> 
>

Sorry. I was taken ill for weeks and forgot to follow these discussions.
Especially I should say sorry to Oleg.

I have different concept. cpuset_cpus_allowed() is not called at atomic
context nor non-preemptable context nor other critical context.
So it should be allowed to use mutexs. That's what I think.

There is a bug when migration_call() requires a mutex
before migration has been finished when cpu offline as Oleg described.

Bug this bug is only happened when cpu offline. cpu offline is rare and
is slowpath. I think we should fix cpu offline and ensure it requires
the mutex safely.

Oleg's patch moves all dirty things into CPUSET subsystem and makes
cpuset_cpus_allowed() does not require any mutex and increases CPUSET's
coupling. I don't feel it's good.

Anyway, Oleg's patch works good.

> > cpuset_cpus_allowed() is not only  used for CPU offline.
> > >
> > > sched_setaffinity() also uses it.
> 
> Sure. And it must take get_online_cpus() to avoid the races with hotplug.

Oleg hasn't answered that
"is it safe when pdflush() calls cpuset_cpus_allowed()?".
A patch may be needed to ensure pdflush() calls cpuset_cpus_allowed() safely.

One other minor thing:
Oleg's patch may introduce a trouble in PREEEMPT_RT tree, because
spinlock in RT is also mutex. Likely I'm wrong.

- Lai

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