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

[Peter Zijlstra]

On Fri, Oct 23, 2020 at 05:31:18PM -0400, Joel Fernandes wrote:
> On Fri, Oct 23, 2020 at 09:26:54PM +0200, Peter Zijlstra wrote:

> > 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.

Hummm... Looking at it again I wonder if I can make something like the
below work.

(depends on the next patch that pulls core_forceidle into core-wide
state)

That would retain the CFS-cgroup optimization as well, for as long as
there's no cookies around.

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4691,8 +4691,6 @@ pick_next_task(struct rq *rq, struct tas
 		return next;
 	}
 
-	put_prev_task_balance(rq, prev, rf);
-
 	smt_mask = cpu_smt_mask(cpu);
 
 	/*
@@ -4707,14 +4705,25 @@ pick_next_task(struct rq *rq, struct tas
 	 */
 	rq->core->core_task_seq++;
 	need_sync = !!rq->core->core_cookie;
-
-	/* reset state */
-reset:
-	rq->core->core_cookie = 0UL;
 	if (rq->core->core_forceidle) {
 		need_sync = true;
 		rq->core->core_forceidle = false;
 	}
+
+	if (!need_sync) {
+		next = __pick_next_task(rq, prev, rf);
+		if (!next->core_cookie) {
+			rq->core_pick = NULL;
+			return next;
+		}
+		put_prev_task(next);
+		need_sync = true;
+	} else {
+		put_prev_task_balance(rq, prev, rf);
+	}
+
+	/* reset state */
+	rq->core->core_cookie = 0UL;
 	for_each_cpu(i, smt_mask) {
 		struct rq *rq_i = cpu_rq(i);
 
@@ -4744,35 +4752,8 @@ pick_next_task(struct rq *rq, struct tas
 			 * 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 (i == cpu && !need_sync)
-					goto next_class;
-
+			if (!p)
 				continue;
-			}
-
-			/*
-			 * Optimize the 'normal' case where there aren't any
-			 * cookies and we don't need to sync up.
-			 */
-			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;
-			}
 
 			rq_i->core_pick = p;