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SubjectRe: [RFC 0/12][PATCH] SCHED_DEADLINE: core of the scheduling class
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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@ekiga.net /
dario.faggioli@jabber.org
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