On Thu, 2009-10-08 at 22:59 +0200, Frans Pop wrote:
> On Thursday 08 October 2009, Mike Galbraith wrote:
> > I still see very high latencies coming out of idle (last noted was >
> > 300ms, NO_HZ) with this patch,
> >
> > Like this:
> >
> > Cause Maximum Percentage
> > Scheduler: waiting for cpu 604.2 msec 49.0 %
>
> I'm not seeing anything even remotely like that.
Instrumenting, I saw (stale) clock deltas of >900ms coming out of idle,
delta being the difference between rq->clock when we hit update_curr()
and discover that this queue was idle, and what the clock will be an
instant or two later when somebody winds the clock.
I've been watching latencytop for a while now to make sure latency is
really dead. I see no twitching, so...
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@xxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
LKML-Reference: <new-submission>
---
kernel/sched.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -2311,7 +2311,7 @@ static int try_to_wake_up(struct task_st
{
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_st
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_st
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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