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


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