On Fri, Oct 23, 2020 at 09:26:54PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 23, 2020 at 01:57:24PM -0400, Joel Fernandes wrote:
> > On Fri, Oct 23, 2020 at 03:54:00PM +0200, Peter Zijlstra wrote:
> > > On Fri, Oct 23, 2020 at 03:51:29PM +0200, Peter Zijlstra wrote:
> > > > On Mon, Oct 19, 2020 at 09:43:16PM -0400, Joel Fernandes (Google) wrote:
> > > > > + /*
> > > > > + * If this sibling doesn't yet have a suitable
> > > > > task to
> > > > > + * run; ask for the most elegible task, given
> > > > > the
> > > > > + * highest priority task already selected for
> > > > > this
> > > > > + * core.
> > > > > + */
> > > > > + p = pick_task(rq_i, class, max);
> > > > > + if (!p) {
> > > > > + /*
> > > > > + * If there weren't no cookies; we
> > > > > don't need to
> > > > > + * bother with the other siblings.
> > > > > + * If the rest of the core is not
> > > > > running a tagged
> > > > > + * task, i.e. need_sync == 0, and the
> > > > > current CPU
> > > > > + * which called into the schedule()
> > > > > loop does not
> > > > > + * have any tasks for this class, skip
> > > > > selecting for
> > > > > + * other siblings since there's no
> > > > > point. We don't skip
> > > > > + * for RT/DL because that could make
> > > > > CFS force-idle RT.
> > > > > + */
> > > > > + if (i == cpu && !need_sync && class ==
> > > > > &fair_sched_class)
> > > > > + goto next_class;
> > > > > +
> > > > > + continue;
> > > > > + }
> > > >
> > > > I'm failing to understand the class == &fair_sched_class bit.
> >
> > The last line in the comment explains it "We don't skip for RT/DL because
> > that could make CFS force-idle RT.".
>
> Well, yes, but it does not explain how this can come about, now does it.
Sorry, I should have made it a separate commit with the below explanation. Oh
well, live and learn!
> > Even if need_sync == false, we need to go look at other CPUs (non-local
> > CPUs) to see if they could be running RT.
> >
> > Say the RQs in a particular core look like this:
> > Let CFS1 and CFS2 be 2 tagged CFS tags. Let RT1 be an untagged RT task.
> >
> > rq0 rq1
> > CFS1 (tagged) RT1 (not tag)
> > CFS2 (tagged)
> >
> > Say schedule() runs on rq0. Now, it will enter the above loop and
> > pick_task(RT) will return NULL for 'p'. It will enter the above if() block
> > and see that need_sync == false and will skip RT entirely.
> >
> > The end result of the selection will be (say prio(CFS1) > prio(CFS2)):
> > rq0 rq1
> > CFS1 IDLE
> >
> > When it should have selected:
> > rq0 r1
> > IDLE RT
> >
> > I saw this issue on real-world usecases in ChromeOS where an RT task gets
> > constantly force-idled and breaks RT. The "class == &fair_sched_class" bit
> > cures it.
>
> Ah, I see. The thing is, this looses the optimization for a bunch of
> valid (and arguably common) scenarios. The problem is that the moment we
> end up selecting a task with a cookie we've invalidated the premise
> under which we ended up with the selected task.
>
> How about this then?
This does look better. It makes sense and I think it will work. I will look
more into it and also test it.
BTW, as further optimization in the future, isn't it better for the
schedule() loop on 1 HT to select for all HT *even if* need_sync == false to
begin with? i.e. no cookied tasks are runnable.
That way the pick loop in schedule() running on other HTs can directly pick
what was pre-selected for it via:
if (rq->core->core_pick_seq == rq->core->core_task_seq &&
rq->core->core_pick_seq != rq->core_sched_seq &&
rq->core_pick)
.. which I think is more efficient. Its just a thought and may not be worth
doing.
thanks,
- Joel
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4709,6 +4709,7 @@ pick_next_task(struct rq *rq, struct tas
> need_sync = !!rq->core->core_cookie;
>
> /* reset state */
> +reset:
> rq->core->core_cookie = 0UL;
> for_each_cpu(i, smt_mask) {
> struct rq *rq_i = cpu_rq(i);
> @@ -4748,14 +4749,8 @@ pick_next_task(struct rq *rq, struct tas
> /*
> * If there weren't no cookies; we don't need to
> * bother with the other siblings.
> - * If the rest of the core is not running a
> tagged
> - * task, i.e. need_sync == 0, and the current
> CPU
> - * which called into the schedule() loop does
> not
> - * have any tasks for this class, skip
> selecting for
> - * other siblings since there's no point. We
> don't skip
> - * for RT/DL because that could make CFS
> force-idle RT.
> */
> - if (i == cpu && !need_sync && !p->core_cookie)
> + if (i == cpu && !need_sync)
> goto next_class;
>
> continue;
> @@ -4765,7 +4760,17 @@ pick_next_task(struct rq *rq, struct tas
> * Optimize the 'normal' case where there aren't any
> * cookies and we don't need to sync up.
> */
> - if (i == cpu && !need_sync && !p->core_cookie) {
> + if (i == cpu && !need_sync) {
> + if (p->core_cookie) {
> + /*
> + * This optimization is only valid as
> + * long as there are no cookies
> + * involved.
> + */
> + need_sync = true;
> + goto reset;
> + }
> +
> next = p;
> goto done;
> }
> @@ -4805,7 +4810,6 @@ pick_next_task(struct rq *rq, struct tas
> */
> need_sync = true;
> }
> -
> }
> }
> next_class:;
>
