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Subject[tip:sched/urgent] sched: Move update_curr() in check_preempt_wakeup() to avoid redundant call
Commit-ID:  a65ac745e47e91f9d98dbf07f22ed0492e34d998
Gitweb: http://git.kernel.org/tip/a65ac745e47e91f9d98dbf07f22ed0492e34d998
Author: Jupyung Lee <jupyung@gmail.com>
AuthorDate: Tue, 17 Nov 2009 18:51:40 +0900
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 9 Dec 2009 10:03:06 +0100

sched: Move update_curr() in check_preempt_wakeup() to avoid redundant call

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@gmail.com>
[ Place update_curr() right before the wake_preempt_entity() call, which
is the only thing that relies on the updated vruntime ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1258451500-6714-1-git-send-email-jupyung@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
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 44ec80c..4dec185 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1651,8 +1651,6 @@ 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;
@@ -1710,6 +1708,8 @@ 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);
/*

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