linux-kernel - Re: [PATCH v4 3/3] sched: optimize migration by forcing rmb() and updating to be called once

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Date:	Wed, 18 Nov 2015 08:37:00 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	yuyang.du@...el.com, pjt@...gle.com, efault@....de,
	tglx@...utronix.de
Subject: Re: [PATCH v4 3/3] sched: optimize migration by forcing rmb() and
 updating to be called once

On Tue, Nov 17, 2015 at 12:21:49PM +0100, Peter Zijlstra wrote:
> 
> Looks that way, I'm not sure we always hold pi_lock there. But I'm low
> on sleep, so I could have overlooked something.
> 
> See for example move_queued_task(), we call set_task_cpu() with rq->lock
> held, but no pi_lock.

Indeed.

> 
> > I thought the comment above migrate_task_rq_fair() is correct rather
> > than CONFIG_LOCKDEP comment in set_task_cpu(), when I read it. I think
> > these two comments are conflict each other a little bit, so one of
> > those should be fixed.
> 
> Agreed.

Which one do you think to be fixed? The one above migrate_task_rq_fair()?
I wonder if it would be ok even it does not hold pi_lock in
migrate_task_rq_fair(). If you say *no problem*, I will try to fix the
comment.

> 
> I meant, if you call __set_task_cpu() before
> sched_class::migrate_task_rq(), in that case task_rq_lock() will no
> longer fully serialize against set_task_cpu().
> 
> Because once you've called __set_task_cpu(), task_rq_lock() will acquire
> the _other_ rq->lock. And we cannot rely on our rq->lock to serialize
> things.

I agree with you if migtrate_task_rq() can be serialized by rq->lock
without holding pi_lock. (even though I am still wondering..)

But I thought it was no problem if migrate_task_rq() was serialized only
by pi_lock as the comment above the migrate_task_rq() describes, because
breaking rq->lock does not affect the sericalization by pi_lock.

I would appreciate it if you would answer my questions.

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