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

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Date:	Fri, 13 May 2011 16:33:25 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Yong Zhang <yong.zhang0@...il.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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

Hello,

>>>> (3) schedule() makes fallback and cpuset_cpus_allowed_fallback
>>>>      change task->cpus_allowed
>>>
>>> I'm failing to see how this is happening, surely that kthread isn't
>>> actually running that early?
>>
>> If my understand correctly, current call graph is below.
>>
>> kernel_init()
>>         smp_init();
>>                 cpu_up()
>>                         ... cpu hotplug notification
>>                                 kthread_create()
>>         sched_init_smp();
>>
>>
>> So, cpu hotplug event is happen before sched_init_smp(). The old rule is,
>> all kthreads of using cpu-up notification have to use kthread_bind(). It
>> protected from sched load balance.
>>
>> but, now cpuset_cpus_allowed_fallback() forcedly change kthread's cpumask.
>> Why is this works? the point are two.
>>
>> - 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.

Hmm..

I haven't catch your point. cpu_up() call both CPU_UP_PREPARE and CPU_ONLINE
notification. Thus, CPU_ONLINE still be called before sched_init_smp().

Am I missing something?


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