sched: fix ideal_runtime calculations for reniced tasks

fix ideal_runtime:

  - do not scale it using niced_granularity()
    it is against sum_exec_delta, so its wall-time, not fair-time.

  - move the whole check into __check_preempt_curr_fair()
    so that wakeup preemption can also benefit from the new logic.

this also results in code size reduction:

   text    data     bss     dec     hex filename
  13391     228    1204   14823    39e7 sched.o.before
  13369     228    1204   14801    39d1 sched.o.after
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_fair.c

@@ -678,11 +678,31 @@ __check_preempt_curr_fair(struct cfs_rq *cfs_rq, struct sched_entity *se,
 			  struct sched_entity *curr, unsigned long granularity)
 {
 	s64 __delta = curr->fair_key - se->fair_key;
+	unsigned long ideal_runtime, delta_exec;
+
+	/*
+	 * ideal_runtime is compared against sum_exec_runtime, which is
+	 * walltime, hence do not scale.
+	 */
+	ideal_runtime = max(sysctl_sched_latency / cfs_rq->nr_running,
+			(unsigned long)sysctl_sched_min_granularity);
+
+	/*
+	 * If we executed more than what the latency constraint suggests,
+	 * reduce the rescheduling granularity. This way the total latency
+	 * of how much a task is not scheduled converges to
+	 * sysctl_sched_latency:
+	 */
+	delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
+	if (delta_exec > ideal_runtime)
+		granularity = 0;
 
 	/*
 	 * Take scheduling granularity into account - do not
 	 * preempt the current task unless the best task has
 	 * a larger than sched_granularity fairness advantage:
+	 *
+	 * scale granularity as key space is in fair_clock.
 	 */
 	if (__delta > niced_granularity(curr, granularity))
 		resched_task(rq_of(cfs_rq)->curr);
@@ -731,7 +751,6 @@ static void put_prev_entity(struct cfs_rq *cfs_rq, struct sched_entity *prev)
 
 static void entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 {
-	unsigned long gran, ideal_runtime, delta_exec;
 	struct sched_entity *next;
 
 	/*
@@ -748,21 +767,8 @@ static void entity_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 	if (next == curr)
 		return;
 
-	gran = sched_granularity(cfs_rq);
-	ideal_runtime = niced_granularity(curr,
-		max(sysctl_sched_latency / cfs_rq->nr_running,
-		    (unsigned long)sysctl_sched_min_granularity));
-	/*
-	 * If we executed more than what the latency constraint suggests,
-	 * reduce the rescheduling granularity. This way the total latency
-	 * of how much a task is not scheduled converges to
-	 * sysctl_sched_latency:
-	 */
-	delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
-	if (delta_exec > ideal_runtime)
-		gran = 0;
-
-	__check_preempt_curr_fair(cfs_rq, next, curr, gran);
+	__check_preempt_curr_fair(cfs_rq, next, curr,
+			sched_granularity(cfs_rq));
 }
 
 /**************************************************