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

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Date:	Tue, 29 Dec 2009 15:30:42 +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 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).

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.



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