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

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Date:	Tue, 29 Dec 2009 15:40:11 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Raistlin <raistlin@...ux.it>
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>,
	Bjoern Brandenburg <bbb@...unc.edu>,
	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 15:37 +0100, Peter Zijlstra wrote:
> 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).
> 
> Hmm I suppose we could ignore all that CONFIG_SCHED_HRTICK stuff and
> simply bounce the schedule event using the resched ipi when we find
> we're on the wrong cpu. Not ideal though.. frigging mess all this :-)

Hmm bugger that, that's not going to work since the fallback timer stuff
doesn't run from hardirq context and is generally useless anyway :-)

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