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

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Date:	Thu, 11 Feb 2016 12:12:57 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	mingo@...hat.com, luca.abeni@...tn.it, vincent.guittot@...aro.org,
	wanpeng.li@...mail.com
Subject: Re: [PATCH 1/2] sched/deadline: add per rq tracking of admitted
 bandwidth

On 10/02/16 16:27, Juri Lelli wrote:
> On 10/02/16 09:37, Steven Rostedt wrote:
> > On Wed, 10 Feb 2016 11:32:58 +0000
> > Juri Lelli <juri.lelli@....com> wrote:
> > 

[...]

> > 
> > I applied this patch and patch 2 and hit this:
> > 

[...]

> > 
> > It's the warning you added in __dl_sub_ac().
> > 
> 
> OK. There are still holes where we fail to properly update per-rq bw. It
> seems (by running you test) that we fail to move the per-rq bw when we
> move the root_domain bw due css_set_move_task(). So, the final
> task_dead_dl() tries to remove bw from where there isn't.
> 
> I'm trying to see how we can close this hole.
> 

So, just to give an update from yesterday (kind of tricky this one :/).

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.

Best,

- Juri