LinuxLists.cc - [BUG] How to get real-time priority using idle priority

by Ingo Molnar

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Subject: Re: [BUG] How to get real-time priority using idle priority

* Peter Zijlstra <[email protected]> wrote:

> On Thu, 2009-01-15 at 13:54 +0100, Ingo Molnar wrote:
> > --- a/kernel/sched_fair.c
> > +++ b/kernel/sched_fair.c
> > @@ -283,10 +283,7 @@ static void update_min_vruntime(struct cfs_rq *cfs_rq)
> > struct sched_entity,
> > run_node);
> >
> > - if (vruntime == cfs_rq->min_vruntime)
> > - vruntime = se->vruntime;
> > - else
> > - vruntime = min_vruntime(vruntime, se->vruntime);
> > + vruntime = min_vruntime(vruntime, se->vruntime);
> > }
> >
> > cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, vruntime);
>
> That's wrong

Please send a patch that makes it right, the twists and turns of patches
in this thread is quite confusing so i probably picked up the
last-but-good one. Thanks,

Ingo

2009-01-15 13:17:44

by Peter Zijlstra

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Subject: Re: [BUG] How to get real-time priority using idle priority

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 <[email protected]>
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 <[email protected]>
Tested-by: Mike Galbraith <[email protected]>
Acked-by: Mike Galbraith <[email protected]>
---
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);