linux-kernel - Re: [patch 2/8] track highest prio queued on runqueue

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Date:	Sat, 20 Oct 2007 22:19:11 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Dmitry Adamushko <dmitry.adamushko@...il.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Gregory Haskins <ghaskins@...ell.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch 2/8] track highest prio queued on runqueue


--
On Sat, 20 Oct 2007, Dmitry Adamushko wrote:

> On 19/10/2007, Steven Rostedt <rostedt@...dmis.org> wrote:
> > [ ... ]
> >
> ===================================================================
> > --- linux-test.git.orig/kernel/sched.c  2007-10-19 12:33:09.000000000 -0400
> > +++ linux-test.git/kernel/sched.c       2007-10-19 12:34:32.000000000 -0400
> > @@ -324,6 +324,8 @@ struct rq {
> >         int push_cpu;
> >         /* cpu of this runqueue: */
> >         int cpu;
> > +       /* highest queued rt task prio */
> > +       int highest_prio;
>
> again, could it be moved to 'struct rt_rq' ?
> (so we want to cache it as we don't want to trash more per-cpu bytes
> calling smth like
> if (!rt_nr_running) sched_find_first_bit() from other CPUs)

Thanks Dmitry, I'll look into moving these around.

>
>
> > @@ -972,6 +974,8 @@ static void activate_task(struct rq *rq,
> >
> >         enqueue_task(rq, p, wakeup);
> >         inc_nr_running(p, rq);
> > +
> > +       rq_prio_add_task(rq, p);
> >  }
> >
> >  /*
> > @@ -984,6 +988,8 @@ static void deactivate_task(struct rq *r
> >
> >         dequeue_task(rq, p, sleep);
> >         dec_nr_running(p, rq);
> > +
> > +       rq_prio_remove_task(rq, p);
> >  }
>
> {enqueue,dequeue}_task_rt() would be more appropriate places.

Yep, I already have that done in my second queue (not yet posted).

>
>
> > +static inline void rq_prio_add_task(struct rq *rq, struct task_struct *p)
> > +{
> > +       if (unlikely(rt_task(p)) && p->prio < rq->highest_prio)
> > +               rq->highest_prio = p->prio;
> > +}
> > +
> > +static inline void rq_prio_remove_task(struct rq *rq, struct task_struct *p)
> > +{
> > +       struct rt_prio_array *array;
> > +
> > +       if (unlikely(rt_task(p))) {
> > +               if (rq->rt_nr_running) {
> > +                       if (p->prio >= rq->highest_prio) {
> > +                               /* recalculate */
> > +                               array = &rq->rt.active;
> > +                               rq->highest_prio =
> > +                                       sched_find_first_bit(array->bitmap);
> > +                       } /* otherwise leave rq->highest prio alone */
> > +               } else
> > +                       rq->highest_prio = MAX_RT_PRIO;
> > +       }
> > +}
> > +#endif /* CONFIG_SMP */
> > +
>
> (just a few thoughts)
>
> we call sched_find_first_bit() in pick_next_task_rt() in the case when
> rt_nr_running != 0.
>
> So if we can tolerate the 'latency' of updating the 'highest_prio' ==
> the interval of time between deactivate_task() and pick_next_task() in
> schedule() then rq_prio_remove_task() would just need to do a single
> thing:
>
> /* No more RT tasks: */
> if (!rt_nr_running)
>         highest_prio = MAX_RT_PRIO;
>
> and then,
>
> static struct task_struct *pick_next_task_rt(struct rq *rq)
> {
>         struct rt_prio_array *array = &rq->rt.active;
>         struct task_struct *next;
>         struct list_head *queue;
>         int idx;
>
> -        idx = sched_find_first_bit(array->bitmap);
> +       rq->highest_prio = idx = sched_find_first_bit(array->bitmap);
>
> [ ... ]
>
> additionally, if we can tolerate the 'latency' (of updating
> highest_prio) == the worst case scheduling latency, then
> rq_prio_add_task() is not necessary at all.

In my logging of test runs, having this 'latency' of highest_prio caused
missed migrations. I tried various things to do like what you said, but
they failed the rt-migrate-test program.

Seemed like the only place to modify highest_prio is from the queue and
dequeue.  Othrewise, my tests failed.

Thanks!

-- Steve

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