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.
> 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?
--- 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:;
