linux-kernel - Re: [PATCH 1/2] cpuset: fix cpuset_cpus_allowed_fallback() don't update tsk->rt.nr_cpus

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Date:	Sat, 14 May 2011 20:17:16 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Yong Zhang <yong.zhang0@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Li Zefan <lizf@...fujitsu.com>,
	Miao Xie <miaox@...fujitsu.com>
Subject: Re: [PATCH 1/2] cpuset: fix cpuset_cpus_allowed_fallback() don't
 update tsk->rt.nr_cpus_allowed

2011/5/14 Peter Zijlstra <a.p.zijlstra@...llo.nl>:
> On Fri, 2011-05-13 at 14:42 +0800, Yong Zhang wrote:
>> > - rcu_cpu_kthread_should_stop() call set_cpus_allowed_ptr() again
>> > periodically.
>> >  then, it can reset cpumask if cpuset_cpus_allowed_fallback() change it.
>> >  my debug print obseve following cpumask change occur at boot time.
>> >     1) kthread_bind: bind cpu1
>> >     2) cpuset_cpus_allowed_fallback: bind possible cpu
>> >     3) rcu_cpu_kthread_should_stop: rebind cpu1
>> > - while tsk->rt.nr_cpus_allowed == 1, sched load balancer never be crash.
>>
>> Seems rcu_spawn_one_cpu_kthread() call wake_up_process() directly,
>> which is under hotplug event CPU_UP_PREPARE. Maybe it should be
>> under CPU_ONLINE.
>
> IIRC I talked to Paul about this a while back and ONLINE is too late,
> however STARTING should work. At the time he couldn't quite get that to
> work, but the above situation is indeed the root cause of our problems.
>
> We shouldn't try to run a cpu affine thread before the cpu in question
> is actually able to run stuff.
>
> I did me a little hackery and with the below patch my kernel still
> boots...
>
> Would that sort your issue?

Great!!
Unfortunately, I can't test this until next wednesday. but I'll do it
as far as possible soon.

Thanks.
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