Re: [PATCH v2] sched/fair: Forfeit vruntime on yield

[Peter Zijlstra]

On Thu, Sep 18, 2025 at 08:43:00AM +0200, Peter Zijlstra wrote:
> On Tue, Sep 16, 2025 at 06:00:35PM +0200, Fernand Sieber wrote:
> > After further testing I think we should stick with the original approach or
> > iterate on the vruntime forfeiting.
> > 
> > The vruntime forfeiting doesn't work well with core scheduling. The core
> > scheduler picks the best task across the SMT mask, and then the siblings run a
> > matching task no matter what. This means the core scheduler can keep picking
> > the yielding task on the sibling even after it becomes ineligible (because it's
> > preferrable than force idle). In this scenario the vruntime of the yielding
> > task runs away rapidly, which causes problematic imbalances later on.
> > 
> > Perhaps an alternative is to forfeit the vruntime (set it to the deadline), but
> > only once. I.e don't do it if the task is already ineligible? If the task is
> > ineligible then we simply increment the deadline as in my original patch?
> > 
> > Peter, let me know your thoughts on this.
> 
> Sorry, I missed this email earlier. I'll go ponder it a bit -- my brain
> is esp. slow today due to a cold :/

Right; so you're saying something like the below, right?

Yeah, I suppose we can do that; please write a coherent comment on it
though, so we can remember why, later on.

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5c94caa93085..e75abf3c256d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9012,8 +9012,13 @@ static void yield_task_fair(struct rq *rq)
 	 */
 	rq_clock_skip_update(rq);
 
-	se->vruntime = se->deadline;
-	se->deadline += calc_delta_fair(se->slice, se);
+	/*
+	 * comment...
+	 */
+	if (entity_eligible(cfs_rq, se)) {
+		se->vruntime = se->deadline;
+		se->deadline += calc_delta_fair(se->slice, se);
+	}
 }
 
 static bool yield_to_task_fair(struct rq *rq, struct task_struct *p)