linux-kernel - Re: [RFC 0/12][PATCH] SCHED_DEADLINE: core of the scheduling class

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Date:	Wed, 13 Jan 2010 17:32:21 +0100
From:	Dario Faggioli <faggioli@...dalf.sssup.it>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	michael trimarchi <michael@...dence.eu.com>,
	Fabio Checconi <fabio@...dalf.sssup.it>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Johan Eker <johan.eker@...csson.com>,
	"p.faure" <p.faure@...tech.ch>,
	Chris Friesen <cfriesen@...tel.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Henrik Austad <henrik@...tad.us>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Darren Hart <darren@...art.com>,
	Sven-Thorsten Dietrich <sven@...bigcorporation.com>,
	Claudio Scordino <claudio@...dence.eu.com>,
	Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
	"giuseppe.lipari" <giuseppe.lipari@...up.it>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [RFC 0/12][PATCH] SCHED_DEADLINE: core of the scheduling class

On Tue, 2009-12-29 at 15:30 +0100, Peter Zijlstra wrote:
> On Fri, 2009-10-16 at 17:40 +0200, Raistlin wrote:
> > +struct task_struct *pick_next_task_deadline(struct rq *rq)
> > +{
> > +       struct sched_dl_entity *dl_se;
> > +       struct task_struct *p;
> > +       struct dl_rq *dl_rq;
> > +
> > +       dl_rq = &rq->dl;
> > +
> > +       if (likely(!dl_rq->dl_nr_running))
> > +               return NULL;
> > +
> > +       dl_se = pick_next_deadline_entity(rq, dl_rq);
> > +       BUG_ON(!dl_se);
> > +
> > +       p = deadline_task_of(dl_se);
> > +       p->se.exec_start = rq->clock;
> > +#ifdef CONFIG_SCHED_HRTICK
> > +       if (hrtick_enabled(rq))
> > +               start_hrtick_deadline(rq, p);
> > +#endif
> > +       return p;
> > +} 
> 
> I'm not sure about actually using hrtick like this, I'd expect
> SCHED_DEADLINE to always use hrtimers when available.  The only reason
> to use some of the hrtick infrastructure is to re-use the hrtick_start()
> logic which uses IPIs to ensure we program the timer on the right cpu
> (so we can schedule from it).
> 
Yeah, that and the fact that it seemed to me very easy and clean to:
- check for runtime enforcement inside the task_tick_deadline function,
  as other scheduling classes do, and then
- if possible, ask that task_tick_deadline function to be called right 
  at the time instant I expect my runtime to be depleted. If that won't 
  happen --because of no-hrtick or no-hires-hrtimers-- the check will
  still be performed during the next tick.

> The whole IPI mess requires USE_GENERIC_SMP_HELPERS, which makes
> CONFIG_HRTICK useful (ensures we have hrtimers enabled and have generic
> IPI bits)
> 
> The problem is that things like hrtick_enabled() also check
> sched_feat(HRTICK) which is disabled by default (because programming the
> clock hw on each schedule was found too expensive) but that should not
> stop SCHED_DEADLINE from using it.
> 
Mmm... I might have lost you here... :-(

Do you think that keep using hrtick_start and alike, even if
sched_feat(HRTICK) is disabled, could be good enough? Or are you
suggesting something different?
IOTH, should I simply bypass the sched_feat()/hrtick_enabled() check or
you think I need something more?

Thanks and regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
----------------------------------------------------------------------
Dario Faggioli, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa  (Italy)

http://blog.linux.it/raistlin / raistlin@...ga.net /
dario.faggioli@...ber.org

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