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@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/CAPM31R+2Ke2urUZKao5W92_LupdR4AYEv-EZWiJ3tG=tEes2cw@mail.gmail.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index e7d67a9e259a..f88720b3df8b 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) {
                 /*