[tip:sched/urgent] sched: Update the clock of runqueue select_task_rq() selected

[tip-bot for Mike Galbraith <efault@gmx.de>]

Commit-ID:  f5dc37530ba8a35aae0f7f4f13781d1904f71e94
Gitweb:     http://git.kernel.org/tip/f5dc37530ba8a35aae0f7f4f13781d1904f71e94
Author:     Mike Galbraith <efault@gmx.de>
AuthorDate: Fri, 9 Oct 2009 08:35:03 +0200
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 9 Oct 2009 15:58:11 +0200

sched: Update the clock of runqueue select_task_rq() selected

In try_to_wake_up(), we update the runqueue clock, but
select_task_rq() may select a different runqueue than the one we
updated, leaving the new runqueue's clock stale for a bit.

This patch cures occasional huge latencies reported by latencytop
when coming out of idle on a mostly idle NO_HZ box.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1255070103.7639.30.camel@marge.simson.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/sched.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 76c0e96..e3f8f4f 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2311,7 +2311,7 @@ static int try_to_wake_up(struct task_struct *p, unsigned int state,
 {
 	int cpu, orig_cpu, this_cpu, success = 0;
 	unsigned long flags;
-	struct rq *rq;
+	struct rq *rq, *orig_rq;
 
 	if (!sched_feat(SYNC_WAKEUPS))
 		wake_flags &= ~WF_SYNC;
@@ -2319,7 +2319,7 @@ static int try_to_wake_up(struct task_struct *p, unsigned int state,
 	this_cpu = get_cpu();
 
 	smp_wmb();
-	rq = task_rq_lock(p, &flags);
+	rq = orig_rq = task_rq_lock(p, &flags);
 	update_rq_clock(rq);
 	if (!(p->state & state))
 		goto out;
@@ -2350,6 +2350,10 @@ static int try_to_wake_up(struct task_struct *p, unsigned int state,
 		set_task_cpu(p, cpu);
 
 	rq = task_rq_lock(p, &flags);
+
+	if (rq != orig_rq)
+		update_rq_clock(rq);
+
 	WARN_ON(p->state != TASK_WAKING);
 	cpu = task_cpu(p);