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SubjectRe: [BUG] How to get real-time priority using idle priority
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If you then fold this on top we have 3 patches, alas not along the lines
that would have been pretty:

- update_min_vruntime fix
- SCHED_IDLE weight change
- SCHED_IDLE vs SCHED_OTHER isolation

But I guess that's life.

---
Subject: sched: fix update_min_vruntime
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Thu Jan 15 14:13:28 CET 2009

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>
---
kernel/sched_fair.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c
@@ -283,7 +283,10 @@ static void update_min_vruntime(struct c
struct sched_entity,
run_node);

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

cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, vruntime);



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