Re: [PATCH -tip 02/32] sched: Introduce sched_class::pick_task()

[Peter Zijlstra]

On Wed, Nov 25, 2020 at 05:28:36PM +0100, Vincent Guittot wrote:
> On Wed, 18 Nov 2020 at 00:20, Joel Fernandes (Google)

> > +#ifdef CONFIG_SMP
> > +static struct task_struct *pick_task_fair(struct rq *rq)
> > +{
> > +       struct cfs_rq *cfs_rq = &rq->cfs;
> > +       struct sched_entity *se;
> > +
> > +       if (!cfs_rq->nr_running)
> > +               return NULL;
> > +
> > +       do {
> > +               struct sched_entity *curr = cfs_rq->curr;
> > +
> > +               se = pick_next_entity(cfs_rq, NULL);
> 
> Calling pick_next_entity clears buddies. This is fine without
> coresched because the se will be the next one. But calling
> pick_task_fair doesn't mean that the se will be used

Urgh, nice one :/

> > +
> > +               if (curr) {
> > +                       if (se && curr->on_rq)
> > +                               update_curr(cfs_rq);
> > +
> 
> Shouldn't you check if cfs_rq is throttled ?

Hmm,... I suppose we do.

> > +                       if (!se || entity_before(curr, se))
> > +                               se = curr;
> > +               }
> > +
> > +               cfs_rq = group_cfs_rq(se);
> > +       } while (cfs_rq);
> > +
> > +       return task_of(se);
> > +}
> > +#endif

Something like so then?

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4354,6 +4354,8 @@ check_preempt_tick(struct cfs_rq *cfs_rq
 static void
 set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
+	clear_buddies(cfs_rq, se);
+
 	/* 'current' is not kept within the tree. */
 	if (se->on_rq) {
 		/*
@@ -4440,8 +4442,6 @@ pick_next_entity(struct cfs_rq *cfs_rq,
 		se = cfs_rq->last;
 	}
 
-	clear_buddies(cfs_rq, se);
-
 	return se;
 }
 
@@ -6982,20 +6982,29 @@ static void check_preempt_wakeup(struct
 #ifdef CONFIG_SMP
 static struct task_struct *pick_task_fair(struct rq *rq)
 {
-	struct cfs_rq *cfs_rq = &rq->cfs;
 	struct sched_entity *se;
-
+	struct cfs_rq *cfs_rq;
+       
+again:
+	cfs_rq = &rq->cfs;
 	if (!cfs_rq->nr_running)
 		return NULL;
 
 	do {
 		struct sched_entity *curr = cfs_rq->curr;
 
-		if (curr && curr->on_rq)
-			update_curr(cfs_rq);
+		/* When we pick for a remote RQ, we'll not have done put_prev_entity() */
+		if (curr) {
+			if (curr->on_rq)
+				update_curr(cfs_rq);
+			else
+				curr = NULL;
 
-		se = pick_next_entity(cfs_rq, curr);
+			if (unlikely(check_cfs_rq_runtime(cfs_rq)))
+				goto again;
+		}
 
+		se = pick_next_entity(cfs_rq, curr);
 		cfs_rq = group_cfs_rq(se);
 	} while (cfs_rq);