linux-kernel - Re: [RFC PATCH v3 7/9] yield_to(), cpu-hotplug: Prevent offlining of other CPUs properly

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Date:	Mon, 10 Dec 2012 01:27:58 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, tj@...nel.org, sbw@....edu,
	amit.kucheria@...aro.org, rostedt@...dmis.org, rjw@...k.pl,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v3 7/9] yield_to(), cpu-hotplug: Prevent offlining
 of other CPUs properly

On 12/10/2012 01:18 AM, Oleg Nesterov wrote:
> On 12/07, Srivatsa S. Bhat wrote:
>>
>> Once stop_machine() is gone from the CPU offline path, we won't be able to
>> depend on local_irq_save() to prevent CPUs from going offline from under us.
> 
> OK, I guess we need to avoid resched_task()->smp_send_reschedule()
> after __cpu_disable() and before migrate_tasks().
> 

Yes.

> But, whatever problem we have,
> 
>> Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going offline,
>> while invoking from atomic context.
> 
> it should be solved, so...
> 
>> -		if (preempt && rq != p_rq)
>> +		if (preempt && rq != p_rq && cpu_online(task_cpu(p)))
> 
> Why do we need this change?
> 
> Afaics, you could add BUG_ON(!cpu_online(...)) instead?
> 
> I am just curious.
>

Oh, I think that's a remnant of v1 (which needed readers to use
cpu_online_stable()). You're right, we don't need it. Or we could put a
BUG_ON instead, like you suggested.

Regards,
Srivatsa S. Bhat

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