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SubjectRe: scheduler nice 19 versus 'idle' behavior / static low-priority scheduling
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On Fri, 2009-01-30 at 01:52 -0500, Nathanael Hoyle wrote:
> On Fri, 2009-01-30 at 07:24 +0100, Mike Galbraith wrote:

> > Sounds like a problem was recently fixed. Can you try 2.6.29-rc3 or
> > 2.6.28.2?
> >
>
> I will try to do so as soon as I get the chance. Do you have any
> specific info on the problem that you believe was fixed and/or the fix
> applied.

The commit text below also applies to +nice tasks.

commit 046e7f77d734778a3b2e7d51ce63da3dbe7a8168
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Thu Jan 15 14:53:39 2009 +0100

sched: fix update_min_vruntime

commit e17036dac189dd034c092a91df56aa740db7146d upstream.

Impact: fix SCHED_IDLE latency problems

OK, so we have 1 running task A (which is obviously curr and the tree is
equally obviously empty).

'A' nicely chugs along, doing its thing, carrying min_vruntime along as it
goes.

Then some whacko speed freak SCHED_IDLE task gets inserted due to SMP
balancing, which is very likely far right, in that case

update_curr
update_min_vruntime
cfs_rq->rb_leftmost := true (the crazy task sitting in a tree)
vruntime = se->vruntime

and voila, min_vruntime is waaay right of where it ought to be.

OK, so why did I write it like that to begin with...

Aah, yes.

Say we've just dequeued current

schedule
deactivate_task(prev)
dequeue_entity
update_min_vruntime

Then we'll set

vruntime = cfs_rq->min_vruntime;

we find !cfs_rq->curr, but do find someone in the tree. Then we _must_
do vruntime = se->vruntime, because

vruntime = min_vruntime(vruntime := cfs_rq->min_vruntime, se->vruntime)

will not advance vruntime, and cause lags the other way around (which we
fixed with that initial patch: 1af5f730fc1bf7c62ec9fb2d307206e18bf40a69
(sched: more accurate min_vruntime accounting).

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Tested-by: Mike Galbraith <efault@gmx.de>
Acked-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 98345e4..06a68c4 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -283,7 +283,7 @@ static void update_min_vruntime(struct cfs_rq *cfs_rq)
struct sched_entity,
run_node);

- if (vruntime == cfs_rq->min_vruntime)
+ if (!cfs_rq->curr)
vruntime = se->vruntime;
else
vruntime = min_vruntime(vruntime, se->vruntime);



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