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SubjectRe: [patch 3/3] sched: move sched_avg_update() to update_cpu_load()
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On Mon, 2010-08-16 at 12:31 -0700, Peter Zijlstra wrote:
> On Mon, 2010-08-16 at 10:46 -0700, Suresh Siddha wrote:
>
> > There is no guarantee that the original cpu won't be doing this in
> > parallel with nohz idle load balancing cpu.
>
> Hmm, true.. bugger.
>
> > > > Fix it by moving the sched_avg_update() to more appropriate update_cpu_load()
> > > > where the CFS load gets updated aswell.
> > >
> > > Right, except it breaks things a bit, at the very least you really need
> > > that update right before reading it, otherwise you can end up with >100%
> > > fractions, which are odd indeed ;-)
> >
> > with the patch, the update always happens before reading it. isn't it?
> >
> > update now happens during the scheduler tick (or during nohz load
> > balancing tick). And the load balancer gets triggered with the tick.
> > So the update (at the tick) should happen before reading it (used by
> > load balancing triggered by the tick). Am I missing something?
>
> We run the load-balancer in softirq context, on -rt that's a task, and
> we could have ran other (more important) RT tasks between the hardirq
> and the softirq running, which would increase the rt_avg and could thus
> result in >100%.
>
> But I think we can simply retain the sched_avg_update(rq) in
> sched_rt_avg_update(), that is ran with rq->lock held and should be
> enough to avoid that case.
>
> We can retain the other bit of you patch, moving sched_avg_update() from
> scale_rt_power() to update_cpu_load(), since that is only concerned with
> lowering the average when there is no actual activity.

Ok. Updated patch appended. Thanks.
---

From: Suresh Siddha <suresh.b.siddha@intel.com>
Subject: sched: move sched_avg_update() to update_cpu_load()

Currently sched_avg_update() (which updates rt_avg stats in the rq) is getting
called from scale_rt_power() (in the load balance context) which doesn't take
rq->lock.

Fix it by moving the sched_avg_update() to more appropriate update_cpu_load()
where the CFS load gets updated aswell.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
---
kernel/sched.c | 3 ++-
kernel/sched_fair.c | 2 --
2 files changed, 2 insertions(+), 3 deletions(-)

Index: tree/kernel/sched_fair.c
===================================================================
--- tree.orig/kernel/sched_fair.c
+++ tree/kernel/sched_fair.c
@@ -2268,8 +2268,6 @@ unsigned long scale_rt_power(int cpu)
struct rq *rq = cpu_rq(cpu);
u64 total, available;

- sched_avg_update(rq);
-
total = sched_avg_period() + (rq->clock - rq->age_stamp);
available = total - rq->rt_avg;

Index: tree/kernel/sched.c
===================================================================
--- tree.orig/kernel/sched.c
+++ tree/kernel/sched.c
@@ -3182,6 +3182,8 @@ static void update_cpu_load(struct rq *t

this_rq->cpu_load[i] = (old_load * (scale - 1) + new_load) >> i;
}
+
+ sched_avg_update(this_rq);
}

static void update_cpu_load_active(struct rq *this_rq)



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