Re: [PATCH v2 2/2] sched/fair: avoid recalculating curr status in place_entity() and requeue_delayed_entity()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Aug 26, 2026 at 01:15:42PM +0300, Kayra Cizmeci wrote:
> In enqueue_task_fair() a bool is calculated by cfs_rq->curr == se.
> But this information gets recalculated on requeue_delayed_entity() and
> requeue_delayed_entity() only gets called in enqueue_task_fair().
> And on place_entity() if se == curr we call the avg_vruntime_weight()
> twice with the same input. place_entity() only gets called in
> enqueue_task_fair() and requeue_delayed_entity().

I found it very hard to follow your Changelog. Could be my Friday brain.
Sorry if I misunderstood.

> @@ -8000,9 +8007,10 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  		util_est_enqueue(cfs_rq, p);
>  
>  	update_curr_eevdf(cfs_rq);
> +	curr = (cfs_rq->curr == se);
>  
>  	if (delayed) {
> -		requeue_delayed_entity(cfs_rq, se);
> +		requeue_delayed_entity(cfs_rq, se, curr);
>  		return;
>  	}
>  
> @@ -8017,18 +8025,18 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	/*
>  	 * XXX comment on the curr thing
>  	 */
> -	curr = (cfs_rq->curr == se);
> +
>  	if (curr)
> -		place_entity(cfs_rq, se, flags);
> +		place_entity(cfs_rq, se, flags, curr);

As I argued here:
  
  https://patch.msgid.link/20260813103155.GC1246887%40noisy.programming.kicks-ass.net

I suspect that case is impossible. Which AFAICT renders your whole patch
one big no-op, no?

That is, I think we want to do the below.

---
 kernel/sched/fair.c | 16 +++-------------
 1 file changed, 3 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4d0b94465d19..3c7fb0684a76 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7996,7 +7996,6 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	struct sched_entity *se = &p->se;
 	struct cfs_rq *cfs_rq = &rq->cfs;
 	unsigned long weight;
-	bool curr;
 
 	if (task_is_throttled(p) && enqueue_throttled_task(p))
 		return;
@@ -8025,23 +8024,14 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	if (p->in_iowait)
 		cpufreq_update_util(rq, SCHED_CPUFREQ_IOWAIT);
 
-	/*
-	 * XXX comment on the curr thing
-	 */
-	curr = (cfs_rq->curr == se);
-	if (curr)
-		place_entity(cfs_rq, se, flags);
 
 	if (se->on_rq && se->sched_delayed)
 		requeue_delayed_entity(cfs_rq, se);
 
 	weight = enqueue_hierarchy(p, flags);
-
-	if (!curr) {
-		reweight_eevdf(cfs_rq, se, weight, false);
-		place_entity(cfs_rq, se, flags | ENQUEUE_QUEUED);
-		__enqueue_entity(cfs_rq, se);
-	}
+	reweight_eevdf(cfs_rq, se, weight, false);
+	place_entity(cfs_rq, se, flags | ENQUEUE_QUEUED);
+	__enqueue_entity(cfs_rq, se);
 
 	if (!rq_h_nr_queued && rq->cfs.h_nr_queued)
 		dl_server_start(&rq->fair_server);