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 23:03:17 +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)

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()

---
Subject: sched: Ensure update_cfs_shares() is called for parents of continuously-running tasks
From: Max Hailperin <max@...tavus.edu>

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
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
@@ -1531,6 +1531,7 @@ static void update_cfs_rq_blocked_load(s
 	}

 	__update_cfs_rq_tg_load_contrib(cfs_rq, force_update);
+	update_cfs_shares(cfs_rq);
 }

 static inline void update_rq_runnable_avg(struct rq *rq, int runnable)
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