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

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Message-ID: <20151117004416.GC18234@byungchulpark-X58A-UD3R>
Date:	Tue, 17 Nov 2015 09:44:16 +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 Mon, Nov 16, 2015 at 01:53:51PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 11, 2015 at 08:51:47AM +0900, Byungchul Park wrote:
> > On Tue, Nov 10, 2015 at 01:16:47PM +0100, Peter Zijlstra wrote:
> > > So the problem is that as soon as that ->cpu store comes through, the
> > > other rq->lock can happen, even though we might still hold a rq->lock
> > > thinking we're serialized.
> > > 
> > > Take for instance move_queued_tasks(), it does:
> > > 
> > > 	dequeue_task(rq, p, 0);
> > > 	p->on_rq = TASK_ON_RQ_MIGRATING;
> > > 	set_task_cpu(p, new_cpu) {
> > > 	  __set_task_cpu();
> > > 
> > > ^^^ here holding rq->lock is insufficient and the below:
> > > 
> > > 	  p->sched_class->migrate_task_rq()
> > 
> > Thank you for explaning in detail, but this's why i asked you.
> 
> > Yes, rq->lock is insufficient in this place as you said, but
> > should migrate_task_rq() be serialized by rq->lock? I might have
> > agreed with you if the migrate_task_rq() should be serialized by
> > rq->lock, but I think it's not the case. I think it would be of
> > if task->pi_lock can work correcly within *if statement* in 
> > set_task_cpu(). Wrong?
> 
> So currently, set_task_cpu() is serialized by:
> 
>  - p->pi_lock; on wakeup
>  - rq->lock; otherwise
> 
> (see the #ifdef CONFIG_LOCKDEP comment in set_task_cpu())

I already read the comment.. Then do you mean the comment above
migrate_task_rq_fair() is wrong and should be fixed? 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.

* the comment above migrate_task_rq_fair() describes it like,
Caller SHOULD HOLD (&p->pi_lock)

* the CONFIG_LOCKDEP comment in set_task_cpu() describes it like,
Caller SHOULD HOLD (&p->pi_lock || &rq->lock)

> 
> This means that sched_class::migrate_task() cannot indeed rely on
> rq->lock for full serialization, however it still means that
> task_rq_lock() will fully serialize against the thing.

Yes I also think this is true.

> 
> By changing this, it no longer will.

???

> 
> Even without that; I think such a change, if correct, is very fragile
> and prone to creating problems later on, and sets bad precedent.
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