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

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Date:	Thu, 11 Feb 2016 12:49:59 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	luca abeni <luca.abeni@...tn.it>
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 11/02/16 13:40, Luca Abeni wrote:
> On Thu, 11 Feb 2016 12:27:54 +0000
> Juri Lelli <juri.lelli@....com> wrote:
> 
> > On 11/02/16 13:22, Luca Abeni wrote:
> > > Hi Juri,
> > > 
> > > On Thu, 11 Feb 2016 12:12:57 +0000
> > > Juri Lelli <juri.lelli@....com> wrote:
> > > [...]
> > > > I think we still have (at least) two problems:
> > > > 
> > > >  - select_task_rq_dl, if we select a different target
> > > >  - select_task_rq might make use of select_fallback_rq, if
> > > > cpus_allowed changed after the task went to sleep
> > > > 
> > > > Second case is what creates the problem here, as we don't update
> > > > task_rq(p) and fallback_cpu ac_bw. I was thinking we might do so,
> > > > maybe adding fallback_cpu in task_struct, from
> > > > migrate_task_rq_dl() (it has to be added yes), but I fear that we
> > > > should hold both rq locks :/.
> > > > 
> > > > 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.

>                 raw_spin_lock(&rq->lock);
>                 add_rq_bw(&p->dl, &rq->dl);
>                 if (migrate_active)
>                         add_running_bw(&p->dl, &rq->dl);
>                 raw_spin_unlock(&rq->lock);
>         }
> [...]
> 
> lockdep is not screaming, and I am not able to trigger any race
> condition or strange behaviour (I am currently at more than 24h of
> continuous stress-testing, but maybe my testcase is not so good in
> finding races here :)
> 

Thanks for sharing what you have!