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 - 17:03:38 EST

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:


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

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
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
@@ -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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