linux-kernel - Re: [PATCH 1/2] sched/deadline: add per rq tracking of admitted bandwidth

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Date:	Thu, 11 Feb 2016 14:05:45 +0100
From:	luca abeni <luca.abeni@...tn.it>
To:	Juri Lelli <juri.lelli@....com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
	wanpeng.li@...mail.com
Subject: Re: [PATCH 1/2] sched/deadline: add per rq tracking of admitted
 bandwidth

On Thu, 11 Feb 2016 12:49:59 +0000
Juri Lelli <juri.lelli@....com> wrote:
[...]
> > > > > Luca, did you already face this problem (if I got it right)
> > > > > and thought of a way to fix it? I'll go back and stare a bit
> > > > > more at those paths.
> > > > In my patch I took care of the first case (modifying
> > > > select_task_rq_dl() to move the utilization from the "old rq"
> > > > to the "new rq"), but I never managed to trigger
> > > > select_fallback_rq() in my tests, so I overlooked that case.
> > > > 
> > > 
> > > Right, I was thinking to do the same. And you did that after
> > > grabbing both locks, right?
> > 
> > Not sure if I did everything correctly, but my code in
> > select_task_rq_dl() currently looks like this (you can obviously
> > ignore the "migrate_active" and "*_running_bw()" parts, and focus on
> > the "*_rq_bw()" stuff):
> > [...]
> >         if (rq != cpu_rq(cpu)) {
> >                 int migrate_active;
> > 
> >                 raw_spin_lock(&rq->lock);
> >                 migrate_active =
> > hrtimer_active(&p->dl.inactive_timer); if (migrate_active) {
> >                         hrtimer_try_to_cancel(&p->dl.inactive_timer);
> >                         sub_running_bw(&p->dl, &rq->dl);
> >                 }
> >                 sub_rq_bw(&p->dl, &rq->dl);
> >                 raw_spin_unlock(&rq->lock);
> >                 rq = cpu_rq(cpu);
> 
> Can't something happen here? My problem is that I use per-rq bw
> tracking to save/restore root_domain state. So, I fear that a
> root_domain update can happen while we are in the middle of moving bw
> from one cpu to another.
Well, I never used the rq utilization to re-build the root_domain
utilization (and I never played with root domains too much... :)...
So, I do not really know. Maybe the code should do:
	raw_spin_lock(&rq->lock);
	raw_spin_lock(&cpu_rq(cpu)->lock);
	sub_rq_bw(&p->dl, &rq->dl);
	add_rq_bw(&p->dl, &cpu_rq(cpu)->dl);
	[...]
?


			Luca