lkml.org 
[lkml]   [2009]   [Oct]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
SubjectRe: [.32-rc3] scheduler: iwlagn consistently high in "waiting for CPU"
From
Date
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@gmx.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
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);




\
 
 \ /
  Last update: 2009-10-09 08:43    [W:0.133 / U:22.452 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site