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>

1 files changed, 6 insertions, 2 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 76c0e9691fc0..e3f8f4f61c8e 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);