[PATCH 1/1] sched: move update_curr() in check_preempt_wakeup() to avoid redundant call

[Jupyung Lee]

Impact: micro-optimization

If a RT task is woken up while a non-RT task is running, 
check_preempt_wakeup() is called to check whether the new task can 
preempt the old task. The function returns quickly without going deeper 
because it is apparent that a RT task can always preempt a non-RT task.

In this situation, check_preempt_wakeup() always calls update_curr() to
update vruntime value of the currently running task. However, the function
call is unnecessary and redundant at that moment because (1) a non-RT task can 
always be preempted by a RT task regardless of its vruntime value, and
(2) update_curr() will be called shortly when the context switch between two occurs.

By moving update_curr() in check_preempt_wakeup(), we can avoid redundant
call to update_curr(), slightly reducing the time taken to wake up RT tasks.

Signed-off-by: Jupyung Lee <jupyung@xxxxxxxxx>
---
 kernel/sched_fair.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 37087a7..32256c0 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1596,8 +1596,6 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
 	int sync = wake_flags & WF_SYNC;
 	int scale = cfs_rq->nr_running >= sched_nr_latency;
 
-	update_curr(cfs_rq);
-
 	if (unlikely(rt_prio(p->prio))) {
 		resched_task(curr);
 		return;
@@ -1655,6 +1653,8 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
 
 	BUG_ON(!pse);
 
+	update_curr(cfs_rq);
+
 	if (wakeup_preempt_entity(se, pse) == 1) {
 		resched_task(curr);
 		/*
-- 
1.6.5.GIT

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