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

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Date:	Thu, 15 Jan 2009 14:16:51 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Mike Galbraith <efault@....de>, Brian Rogers <brian@...w.org>,
	linux-kernel@...r.kernel.org
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 <a.p.zijlstra@...llo.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@...llo.nl>
Tested-by: Mike Galbraith <efault@....de>
Acked-by: Mike Galbraith <efault@....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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