linux-kernel - Re: [RFC][PATCH 12/13] sched/deadline: Introduce deadline servers

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Message-ID: <20190809071306.GL29310@localhost.localdomain>
Date:   Fri, 9 Aug 2019 09:13:06 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
        bristot@...hat.com, balsini@...roid.com, dvyukov@...gle.com,
        tglx@...utronix.de, vpillai@...italocean.com, rostedt@...dmis.org
Subject: Re: [RFC][PATCH 12/13] sched/deadline: Introduce deadline servers

On 08/08/19 12:31, Peter Zijlstra wrote:
> On Thu, Aug 08, 2019 at 10:46:52AM +0200, Juri Lelli wrote:
> > On 08/08/19 10:11, Dietmar Eggemann wrote:
> 
> > > What about the fast path in pick_next_task()?
> > > 
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index bffe849b5a42..f1ea6ae16052 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -3742,6 +3742,9 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
> > >                 if (unlikely(!p))
> > >                         p = idle_sched_class.pick_next_task(rq, prev, rf);
> > >  
> > > +               if (p->sched_class == &fair_sched_class && p->server)
> > > +                       p->server = NULL;
> > > +
> > 
> > Hummm, but then who sets it back to the correct server. AFAIU
> > update_curr() needs a ->server to do the correct DL accounting?
> 
> The above looks ok.
> 
> The pick_next_task_dl() when it selects a server will set ->server.

OK. Yesterday's IRC conversation resolved my perplexity about this
point. I tried to summarize what I've got in the comment below (which we
might want to add on top of Dietmar's fix).

--->8---
 /*
  * Since we are executing the fast path it means that the DL
  * server was throttled and we'll now be scheduled w/o it. So,
  * reset p's pointer to it (accounting won't be performed, as
  * it wouldn't make sense while server is thottled).
  */