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Subject[PATCH] sched/fair: tweak pick_next_entity
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Currently, pick_next_entity(...) has the following structure
(simplified):

[...]
if (last_buddy_ok())
result = last_buddy;
if (next_buddy_ok())
result = next_buddy;
[...]

The intended behavior is to prefer next buddy over last buddy;
the current code somewhat obfuscates this, and also wastes
cycles checking the last buddy when eventually the next buddy is
picked up.

So this patch refactors two 'ifs' above into

[...]
if (next_buddy_ok())
result = next_buddy;
else if (last_buddy_ok())
result = last_buddy;
[...]

Signed-off-by: Peter Oskolkov <posk@google.com>
---
kernel/sched/fair.c | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fc3410b8b990..cec6cf9b2bb3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4465,17 +4465,17 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
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)
+ if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
+ /*
+ * Someone really wants this to run. If it's not unfair, run it.
+ */
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;
+ }

clear_buddies(cfs_rq, se);

--
2.28.0.709.gb0816b6eb0-goog
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