[tip:sched/core] sched: update correct entity's runtime in check_preempt_wakeup()

[tip-bot for Paul Turner]

Commit-ID:  9bbd7374361d9bfc75108c3ad1c1b6db28b1be59
Gitweb:     http://git.kernel.org/tip/9bbd7374361d9bfc75108c3ad1c1b6db28b1be59
Author:     Paul Turner <pjt@xxxxxxxxxx>
AuthorDate: Tue, 5 Jul 2011 19:07:21 -0700
Committer:  Ingo Molnar <mingo@xxxxxxx>
CommitDate: Thu, 21 Jul 2011 18:01:43 +0200

sched: update correct entity's runtime in check_preempt_wakeup()

While looking at check_preempt_wakeup() I realized that we are
potentially updating the wrong entity in the fair-group scheduling
case. In this case the current task's cfs_rq may not be the same as
the one used for the comparison between the waking task and the
existing task's vruntime.

This potentially results in us using a stale vruntime in the
pre-emption decision, providing a small false preference for the
previous task. The effects of this are bounded since we always
perform a hierarchal update on the tick.

Signed-off-by: Paul Turner <pjt@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Link: http://lkml.kernel.org/r/CAPM31R+2Ke2urUZKao5W92_LupdR4AYEv-EZWiJ3tG=tEes2cw@xxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 kernel/sched_fair.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index e7d67a9..f88720b 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1919,8 +1919,8 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
 	if (!sched_feat(WAKEUP_PREEMPT))
 		return;
 
-	update_curr(cfs_rq);
 	find_matching_se(&se, &pse);
+	update_curr(cfs_rq_of(se));
 	BUG_ON(!pse);
 	if (wakeup_preempt_entity(se, pse) == 1) {
 		/*
--
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