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Subject[PATCH v2] sched/fair: enqueue_task_fair optimization
Date
enqueue_task_fair jumps to enqueue_throttle label when cfs_rq_of(se) is
throttled which means that se can't be NULL and we can skip the test.

Reviewed-by: Phil Auld <pauld@redhat.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---

v2 changes:
- Remove useless if statement

kernel/sched/fair.c | 39 ++++++++++++++++++++-------------------
1 file changed, 20 insertions(+), 19 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a0c690d57430..b51b12d63c39 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5513,28 +5513,29 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
list_add_leaf_cfs_rq(cfs_rq);
}

-enqueue_throttle:
- if (!se) {
- add_nr_running(rq, 1);
- /*
- * Since new tasks are assigned an initial util_avg equal to
- * half of the spare capacity of their CPU, tiny tasks have the
- * ability to cross the overutilized threshold, which will
- * result in the load balancer ruining all the task placement
- * done by EAS. As a way to mitigate that effect, do not account
- * for the first enqueue operation of new tasks during the
- * overutilized flag detection.
- *
- * A better way of solving this problem would be to wait for
- * the PELT signals of tasks to converge before taking them
- * into account, but that is not straightforward to implement,
- * and the following generally works well enough in practice.
- */
- if (flags & ENQUEUE_WAKEUP)
- update_overutilized_status(rq);
+ /* At this point se is NULL and we are at root level*/
+ add_nr_running(rq, 1);
+
+ /*
+ * Since new tasks are assigned an initial util_avg equal to
+ * half of the spare capacity of their CPU, tiny tasks have the
+ * ability to cross the overutilized threshold, which will
+ * result in the load balancer ruining all the task placement
+ * done by EAS. As a way to mitigate that effect, do not account
+ * for the first enqueue operation of new tasks during the
+ * overutilized flag detection.
+ *
+ * A better way of solving this problem would be to wait for
+ * the PELT signals of tasks to converge before taking them
+ * into account, but that is not straightforward to implement,
+ * and the following generally works well enough in practice.
+ */
+ if (flags & ENQUEUE_WAKEUP)
+ update_overutilized_status(rq);

}

+enqueue_throttle:
if (cfs_bandwidth_used()) {
/*
* When bandwidth control is enabled; the cfs_rq_throttled()
--
2.17.1
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