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

[Peter Zijlstra]

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:;