linux-kernel - Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM

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Date:	Fri, 27 May 2011 20:53:02 +0530
From:	Santosh Shilimkar <santosh.shilimkar@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Oleg Nesterov <oleg@...hat.com>,
	Marc Zyngier <Marc.Zyngier@....com>,
	Frank Rowand <frank.rowand@...sony.com>,
	linux-kernel@...r.kernel.org, Yong Zhang <yong.zhang0@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()"
 locks up on ARM

Peter,

On 5/26/2011 10:53 PM, Peter Zijlstra wrote:
> On Thu, 2011-05-26 at 19:17 +0200, Peter Zijlstra wrote:
>> On Thu, 2011-05-26 at 19:04 +0200, Oleg Nesterov wrote:
>>> On 05/26, Peter Zijlstra wrote:
>>>>
>>>> @@ -2636,7 +2636,8 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>>>>   		 * to spin on ->on_cpu if p is current, since that would
>>>>   		 * deadlock.
>>>>   		 */
>>>> -		if (p == current) {
>>>> +		if (cpu == smp_processor_id()) {
>>>> +			p->sched_contributes_to_load = 0;
>>>>   			ttwu_queue(p, cpu);
>>>
>>> Btw. I do not pretend I really understand se->vruntime, but in this
>>> case we are doing enqueue_task() without ->task_waking(), however we
>>> pass ENQUEUE_WAKING. Is it correct?
>>
>> No its not, that's the thing that I got wrong the first time and caused
>> these pauses.
>
> We'd end up with something like the below, which isn't too different
> from what I've now got queued.
>
> It has the extra cpu == smp_processor_id() check, but I'm not sure this
> whole case is worth the trouble. I could go stick some counters in to
> verify how often all this happens I guess.
>
Are you planning send version of this patch for stable .39
too ?

Regards
Santosh
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