sched/fair: Fix entity's lag with run to parity

When an entity is enqueued without preempting current, we must ensure
that the slice protection is updated to take into account the slice
duration of the newly enqueued task so that its lag will not exceed
its slice (+ tick).

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250708165630.1948751-6-vincent.guittot@linaro.org

Author: vingu-linaro

kernel/sched/fair.c

@@ -889,13 +889,13 @@ struct sched_entity *__pick_first_entity(struct cfs_rq *cfs_rq)
  * 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)
+static inline void set_protect_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
 	u64 slice = normalized_sysctl_sched_base_slice;
 	u64 vprot = se->deadline;
 
 	if (sched_feat(RUN_TO_PARITY))
-		slice = cfs_rq_min_slice(cfs_rq_of(se));
+		slice = cfs_rq_min_slice(cfs_rq);
 
 	slice = min(slice, se->slice);
 	if (slice != se->slice)
@@ -904,6 +904,13 @@ static inline void set_protect_slice(struct sched_entity *se)
 	se->vprot = vprot;
 }
 
+static inline void update_protect_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
+{
+	u64 slice = cfs_rq_min_slice(cfs_rq);
+
+	se->vprot = min_vruntime(se->vprot, se->vruntime + calc_delta_fair(slice, se));
+}
+
 static inline bool protect_slice(struct sched_entity *se)
 {
 	return ((s64)(se->vprot - se->vruntime) > 0);
@@ -5467,7 +5474,7 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
 		__dequeue_entity(cfs_rq, se);
 		update_load_avg(cfs_rq, se, UPDATE_TG);
 
-		set_protect_slice(se);
+		set_protect_slice(cfs_rq, se);
 	}
 
 	update_stats_curr_start(cfs_rq, se);
@@ -8720,6 +8727,9 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
 	if (__pick_eevdf(cfs_rq, !do_preempt_short) == pse)
 		goto preempt;
 
+	if (sched_feat(RUN_TO_PARITY) && do_preempt_short)
+		update_protect_slice(cfs_rq, se);
+
 	return;
 
 preempt: