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Subject[PATCH v2] sched/fair: Do not set skip buddy up the sched hierarchy
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From: Venkatesh Pallipadi <venki@google.com>

Setting skip buddy all the way up the hierarchy does not play well
with intra-cgroup yield. One typical usecase of yield is when a
thread in a cgroup wants to yield CPU to another thread within the
same cgroup. For such a case, setting the skip buddy all the way up
the hierarchy is counter-productive, as that results in CPU being
yielded to a task in some other cgroup.

So, limit the skip effect only to the task requesting it.

Signed-off-by: Josh Don <joshdon@google.com>
---
v2: Only clear skip buddy on the current cfs_rq

kernel/sched/fair.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 08a233e97a01..0b7a1958ad52 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4051,13 +4051,10 @@ static void __clear_buddies_next(struct sched_entity *se)

static void __clear_buddies_skip(struct sched_entity *se)
{
- for_each_sched_entity(se) {
- struct cfs_rq *cfs_rq = cfs_rq_of(se);
- if (cfs_rq->skip != se)
- break;
+ struct cfs_rq *cfs_rq = cfs_rq_of(se);

+ if (cfs_rq->skip == se)
cfs_rq->skip = NULL;
- }
}

static void clear_buddies(struct cfs_rq *cfs_rq, struct sched_entity *se)
@@ -6552,8 +6549,15 @@ static void set_next_buddy(struct sched_entity *se)

static void set_skip_buddy(struct sched_entity *se)
{
- for_each_sched_entity(se)
- cfs_rq_of(se)->skip = se;
+ /*
+ * One typical usecase of yield is when a thread in a cgroup
+ * wants to yield CPU to another thread within the same cgroup.
+ * For such a case, setting the skip buddy all the way up the
+ * hierarchy is counter-productive, as that results in CPU being
+ * yielded to a task in some other cgroup. So, only set skip
+ * for the task requesting it.
+ */
+ cfs_rq_of(se)->skip = se;
}

/*
--
2.24.0.393.g34dc348eaf-goog
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