sched/fair: Remove sched_feat(START_DEBIT)

With the introduction of avg_vruntime() there is no need to use worse
approximations. Take the 0-lag point as starting point for inserting
new tasks.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230531124603.722361178@infradead.org

Author: Peter Zijlstra

kernel/sched/fair.c

@@ -906,16 +906,6 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	return slice;
 }
 
-/*
- * We calculate the vruntime slice of a to-be-inserted task.
- *
- * vs = s/w
- */
-static u64 sched_vslice(struct cfs_rq *cfs_rq, struct sched_entity *se)
-{
-	return calc_delta_fair(sched_slice(cfs_rq, se), se);
-}
-
 #include "pelt.h"
 #ifdef CONFIG_SMP
 
@@ -4862,16 +4852,7 @@ static inline bool entity_is_long_sleeper(struct sched_entity *se)
 static void
 place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
 {
-	u64 vruntime = cfs_rq->min_vruntime;
-
-	/*
-	 * The 'current' period is already promised to the current tasks,
-	 * however the extra weight of the new task will slow them down a
-	 * little, place the new task so that it fits in the slot that
-	 * stays open at the end.
-	 */
-	if (initial && sched_feat(START_DEBIT))
-		vruntime += sched_vslice(cfs_rq, se);
+	u64 vruntime = avg_vruntime(cfs_rq);
 
 	/* sleeps up to a single latency don't count. */
 	if (!initial) {

kernel/sched/features.h

@@ -6,12 +6,6 @@
  */
 SCHED_FEAT(GENTLE_FAIR_SLEEPERS, true)
 
-/*
- * Place new tasks ahead so that they do not starve already running
- * tasks
- */
-SCHED_FEAT(START_DEBIT, true)
-
 /*
  * Prefer to schedule the task we woke last (assuming it failed
  * wakeup-preemption), since its likely going to consume data we