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.
