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Subject[PATCH v3] sched/fair: Do not set skip buddy up the sched hierarchy
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From: Venkatesh Pallipadi <venki@google.com>

Setting the skip buddy up the hierarchy effectively means that a thread
calling yield will also end up skipping the other tasks in its hierarchy
as well. However, a yielding thread shouldn't end up causing this
behavior on behalf of its entire hierarchy.

For typical uses of yield, setting the skip buddy 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.

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

v3: Modify comment describing the justification for this change.

kernel/sched/fair.c | 15 ++++++++-------
1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 08a233e97a01..0056b57d52cb 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 +6549,12 @@ 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;
+ /*
+ * Only set the skip buddy for the task requesting it. Setting the skip
+ * buddy up the hierarchy would result in skipping all other tasks in
+ * the hierarchy as well.
+ */
+ cfs_rq_of(se)->skip = se;
}

/*
--
2.24.1.735.g03f4e72817-goog
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