Re: [PATCH v8 -tip 06/26] sched: Add core wide task selection and scheduling.

[Peter Zijlstra]

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.
> 
> IIRC the condition is such that the core doesn't have a cookie (we don't
> need to sync the threads) so we'll only do a pick for our local CPU.
> 
> That should be invariant of class.

That is; it should be the exact counterpart of this bit:

> +			/*
> +			 * 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) {
> +				next = p;
> +				goto done;
> +			}

If there is no task found in this class, try the next class, if there
is, we done.