sched/fair: Limit run to parity to the min slice of enqueued entities

Run to parity ensures that current will get a chance to run its full
slice in one go but this can create large latency and/or lag for
entities with shorter slice that have exhausted their previous slice
and wait to run their next slice.

Clamp the run to parity to the shortest slice of all enqueued entities.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250708165630.1948751-5-vincent.guittot@linaro.org

Author: vingu-linaro

kernel/sched/fair.c

@@ -884,18 +884,20 @@ struct sched_entity *__pick_first_entity(struct cfs_rq *cfs_rq)
 /*
  * Set the vruntime up to which an entity can run before looking
  * for another entity to pick.
- * In case of run to parity, we protect the entity up to its deadline.
+ * In case of run to parity, we use the shortest slice of the enqueued
+ * entities to set the protected period.
  * When run to parity is disabled, we give a minimum quantum to the running
  * entity to ensure progress.
  */
 static inline void set_protect_slice(struct sched_entity *se)
 {
-	u64 slice = se->slice;
+	u64 slice = normalized_sysctl_sched_base_slice;
 	u64 vprot = se->deadline;
 
-	if (!sched_feat(RUN_TO_PARITY))
-		slice = min(slice, normalized_sysctl_sched_base_slice);
+	if (sched_feat(RUN_TO_PARITY))
+		slice = cfs_rq_min_slice(cfs_rq_of(se));
 
+	slice = min(slice, se->slice);
 	if (slice != se->slice)
 		vprot = min_vruntime(vprot, se->vruntime + calc_delta_fair(slice, se));