linux-kernel - Re: PROBLEM: Persistent unfair sharing of a processor by auto groups in 3.11-rc2 (has twice regressed)

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Date:	Fri, 26 Jul 2013 23:50:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Turner <pjt@...gle.com>
Cc:	Max Hailperin <max@...tavus.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: PROBLEM: Persistent unfair sharing of a processor by auto groups
 in 3.11-rc2 (has twice regressed)

On Fri, Jul 26, 2013 at 02:24:50PM -0700, Paul Turner wrote:
> On Fri, Jul 26, 2013 at 2:03 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> >
> > OK, so I have the below; however on a second look, Paul, shouldn't that
> > update_cfs_shares() call be in entity_tick(), right after calling
> > update_cfs_rq_blocked_load(). Because placing it in
> > update_cfs_rq_blocked_load() means its now called twice on the
> > enqueue/dequeue paths through:
> >
> >   {en,de}queue_entity()
> >     {en,de}queue_entity_load_avg()
> >       update_cfs_rq_blocked_load()
> >         update_cfs_shares()
> 
> Yes, I agree: placing it directly in entity_tick() would be better.

OK, how about the below then?

> [ In f269ae046 the calls to update_cfs_rq_blocked_load() were amortized
> and the separate update in {en,de}queue_entity_load_avg() were
> removed. ]

Right, I remember/saw that. Did you ever figure out why that regressed;
as in should we look to bring some of that back?

---
Subject: sched: Ensure update_cfs_shares() is called for parents of continuously-running tasks
From: Peter Zijlstra <peterz@...radead.org>
Date: Fri Jul 26 23:48:42 CEST 2013

We typically update a task_group's shares within the dequeue/enqueue
path.  However, continuously running tasks sharing a CPU are not
subject to these updates as they are only put/picked.  Unfortunately,
when we reverted f269ae046 (in 17bc14b7), we lost the augmenting
periodic update that was supposed to account for this; resulting in a
potential loss of fairness.

To fix this, re-introduce the explicit update in
update_cfs_rq_blocked_load() [called via entity_tick()].

Cc: stable@...nel.org
Reported-by: Max Hailperin <max@...tavus.edu>
Reviewed-by: Paul Turner <pjt@...gle.com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
---
 kernel/sched/fair.c |    1 +
 1 file changed, 1 insertion(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2032,6 +2032,7 @@ entity_tick(struct cfs_rq *cfs_rq, struc
 	 */
 	update_entity_load_avg(curr, 1);
 	update_cfs_rq_blocked_load(cfs_rq, 1);
+	update_cfs_shares(cfs_rq);
 
 #ifdef CONFIG_SCHED_HRTICK
 	/*
--
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