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

[Peter Zijlstra <peterz@infradead.org>]

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.