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Subject[PATCH] 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>
---
kernel/sched/fair.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 682a754ea3e1..52ab06585d7f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6647,8 +6647,15 @@ 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;
}

/*
--
2.23.0.700.g56cf767bdb-goog
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