[PATCH 07/30] sched: fix wakeup granularity and buddy granularity

[Peter Zijlstra]

Uncouple buddy selection from wakeup granularity.

The initial idea was that buddies could run ahead as far as a normal task
can - do this by measuring a pair 'slice' just as we do for a normal task.

This means we can drop the wakeup_granularity back to 5ms.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
 kernel/sched.c      |    1 +
 kernel/sched_fair.c |   11 +++++------
 2 files changed, 6 insertions(+), 6 deletions(-)

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c	2008-05-12 00:42:40.000000000 +0200
+++ linux-2.6/kernel/sched.c	2008-05-12 00:43:54.000000000 +0200
@@ -370,6 +370,7 @@
 
 	u64 exec_clock;
 	u64 min_vruntime;
+	u64 pair_start;
 
 	struct rb_root tasks_timeline;
 	struct rb_node *rb_leftmost;
Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c	2008-05-12 00:42:41.000000000 +0200
+++ linux-2.6/kernel/sched_fair.c	2008-05-12 00:47:05.000000000 +0200
@@ -63,13 +63,13 @@ unsigned int __read_mostly sysctl_sched_
 
 /*
  * SCHED_OTHER wake-up granularity.
- * (default: 10 msec * (1 + ilog(ncpus)), units: nanoseconds)
+ * (default: 5 msec * (1 + ilog(ncpus)), units: nanoseconds)
  *
  * This option delays the preemption effects of decoupled workloads
  * and reduces their over-scheduling. Synchronous workloads will still
  * have immediate wakeup/sleep latencies.
  */
-unsigned int sysctl_sched_wakeup_granularity = 10000000UL;
+unsigned int sysctl_sched_wakeup_granularity = 5000000UL;
 
 const_debug unsigned int sysctl_sched_migration_cost = 500000UL;
 
@@ -787,17 +787,16 @@
 	se->prev_sum_exec_runtime = se->sum_exec_runtime;
 }
 
-static int
-wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se);
-
 static struct sched_entity *
 pick_next(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
-	if (!cfs_rq->next)
-		return se;
+	struct rq *rq = rq_of(cfs_rq);
+	u64 pair_slice = rq->clock - cfs_rq->pair_start;
 
-	if (wakeup_preempt_entity(cfs_rq->next, se) != 0)
+	if (!cfs_rq->next || pair_slice > sched_slice(cfs_rq, cfs_rq->next)) {
+		cfs_rq->pair_start = rq->clock;
 		return se;
+	}
 
 	return cfs_rq->next;
 }

-- 

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