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Subject[PATCH] sched: Skip looking at skip if next or last is set
pick_next_entity() prefers next, then last. However code checks if the
left entity can be skipped even if next / last is set.

Check if left entity should be skipped only if next/last is not set.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
kernel/sched/fair.c | 31 +++++++++++++++----------------
1 files changed, 15 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fdee793..cc97b12 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1900,27 +1900,26 @@ static struct sched_entity *pick_next_entity(struct cfs_rq *cfs_rq)
struct sched_entity *left = se;

/*
- * Avoid running the skip buddy, if running something else can
- * be done without getting too unfair.
+ * Someone really wants next to run. If it's not unfair, run it.
*/
- if (cfs_rq->skip == se) {
- struct sched_entity *second = __pick_next_entity(se);
+ if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
+ se = cfs_rq->next;
+ } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
+ /*
+ * Prefer last buddy, try to return the CPU to a preempted
+ * task.
+ */
+ se = cfs_rq->last;
+ } else if (cfs_rq->skip == left) {
+ /*
+ * Avoid running the skip buddy, if running something else
+ * can be done without getting too unfair.
+ */
+ struct sched_entity *second = __pick_next_entity(left);
if (second && wakeup_preempt_entity(second, left) < 1)
se = second;
}

- /*
- * Prefer last buddy, try to return the CPU to a preempted task.
- */
- if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
- se = cfs_rq->last;
-
- /*
- * Someone really wants this to run. If it's not unfair, run it.
- */
- if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
- se = cfs_rq->next;
-
clear_buddies(cfs_rq, se);

return se;


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