linux-kernel - Re: [PATCH 2/4] sched/deadline: Improve admission control for asymmetric CPU capacities

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Date:   Tue, 14 Apr 2020 16:41:19 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        luca abeni <luca.abeni@...tannapisa.it>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Wei Wang <wvw@...gle.com>, Quentin Perret <qperret@...gle.com>,
        Alessio Balsini <balsini@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Morten Rasmussen <morten.rasmussen@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] sched/deadline: Improve admission control for
 asymmetric CPU capacities

On 04/14/20 15:29, Valentin Schneider wrote:
> 
> On 14/04/20 12:40, Qais Yousef wrote:
> >
> > I haven't followed this discussion closely, so I could be missing something
> > here.
> >
> > In sched_cpu_dying() we call set_rq_offline() which clears the cpu in
> > rq->rd->online.
> >
> > So the way I read the code
> >
> >       rd->online = cpumask_and(rd->span, cpu_active_mask)
> >
> > But I could have easily missed some detail.
> >
> 
> sched_cpu_dying() is wayyyy below sched_cpu_deactivate(). This doesn't help
> at all for the dl_cpu_busy() check in sched_cpu_deactivate().

It'd makes more sense to introduce rd->active then rather than a PERCPU global
variable IMHO.

> 
> > Regardless, it seems to me that DL is working around something not right in the
> > definition of rd->span or using the wrong variable.
> >
> 
> What DL is doing now is fine, it only needs to be aligned with the active
> mask (which it is). We're making things a bit trickier by adding capacity
> values into the mix.

I'm still worried that others might be affected by a similar problem.

Will dig deeper next week when I'm back from hols.

Thanks

--
Qais Yousef