Re: PROBLEM: Persistent unfair sharing of a processor by auto groupsin 3.11-rc2 (has twice regressed)

From: Peter Zijlstra
Date: Fri Jul 26 2013 - 17:50:29 EST

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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
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@xxxxxxxxxx
Reported-by: Max Hailperin <max@xxxxxxxxxxxx>
Reviewed-by: Paul Turner <pjt@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
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);

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