Re: [RFC PATCH] sched/eevdf: Return leftmost entity in pick_eevdf() if no eligible entity is found

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Apr 19, 2024 at 10:24:40AM +0200, Peter Zijlstra wrote:
> On Thu, Apr 18, 2024 at 09:03:36PM +0800, Chen Yu wrote:
> 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 31bca05c3612..9f203012e8f5 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -696,15 +696,23 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
> >   *
> >   * XXX could add max_slice to the augmented data to track this.
> >   */
> > +
> > +static s64 limit_entity_lag(struct sched_entity *se, s64 lag)
> > +{
> > +	s64 limit;
> > +
> > +	limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se);
> > +	return clamp(lag, -limit, limit);
> > +}
> 
> Right, helper makes sense.

A possible other helper might be the below. But that depends if we want
to clip after scaling too. No super strong preference.

Please post your patch with a proper changelog and we'll get it sorted.


--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -696,15 +696,19 @@ u64 avg_vruntime(struct cfs_rq *cfs_rq)
  *
  * XXX could add max_slice to the augmented data to track this.
  */
-static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
+static s64 entity_lag(u64 avruntime, struct sched_entity *se)
 {
-	s64 lag, limit;
-
-	SCHED_WARN_ON(!se->on_rq);
-	lag = avg_vruntime(cfs_rq) - se->vruntime;
+	s64 vlag, limit;
 
+	vlag = avruntime - se->vruntime;
 	limit = calc_delta_fair(max_t(u64, 2*se->slice, TICK_NSEC), se);
-	se->vlag = clamp(lag, -limit, limit);
+	return clamp(vlag, -limit, limit);
+}
+
+static void update_entity_lag(struct cfs_rq *cfs_rq, struct sched_entity *se)
+{
+	SCHED_WARN_ON(!se->on_rq);
+	se->vlag = entity_lag(avg_vruntime(cfs_rq), se);
 }
 
 /*
@@ -3760,7 +3764,7 @@ static void reweight_eevdf(struct sched_
 	 *	   = V  - vl'
 	 */
 	if (avruntime != se->vruntime) {
-		vlag = (s64)(avruntime - se->vruntime);
+		vlag = entity_lag(avruntime, se);
 		vlag = div_s64(vlag * old_weight, weight);
 		se->vruntime = avruntime - vlag;
 	}