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);