[RFC PATCH 05/11] sched buddy enable buddy logic starting at 2 running threads

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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Extracted from a "sched-misc-bits.patch" patch from
Peter Zijlstra <a.p.zijlstra@chello.nl>.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 kernel/sched_fair.c |   10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

Index: linux-2.6-lttng.git/kernel/sched_fair.c
===================================================================
--- linux-2.6-lttng.git.orig/kernel/sched_fair.c
+++ linux-2.6-lttng.git/kernel/sched_fair.c
@@ -1647,7 +1647,11 @@ static void check_preempt_wakeup(struct 
 	struct task_struct *curr = rq->curr;
 	struct sched_entity *se = &curr->se, *pse = &p->se;
 	struct cfs_rq *cfs_rq = task_cfs_rq(curr);
-	int scale = cfs_rq->nr_running >= sched_nr_latency;
+	/*
+	 * The buddy logic doesn't work well when there's not actually enough
+	 * tasks for there to be buddies.
+	 */
+	int buddies = (cfs_rq->nr_running >= 2);
 
 	if (unlikely(rt_prio(p->prio)))
 		goto preempt;
@@ -1658,7 +1662,7 @@ static void check_preempt_wakeup(struct 
 	if (unlikely(se == pse))
 		return;
 
-	if (sched_feat(NEXT_BUDDY) && scale && !(wake_flags & WF_FORK))
+	if (sched_feat(NEXT_BUDDY) && buddies && !(wake_flags & WF_FORK))
 		set_next_buddy(pse);
 
 	/*
@@ -1704,7 +1708,7 @@ preempt:
 	if (unlikely(!se->on_rq || curr == rq->idle))
 		return;
 
-	if (sched_feat(LAST_BUDDY) && scale && entity_is_task(se))
+	if (sched_feat(LAST_BUDDY) && buddies && entity_is_task(se))
 		set_last_buddy(se);
 }