Re: [tip:sched/urgent] sched: Fix sched-domain avg_load calculation

[Peter Zijlstra]

On Mon, 2011-04-11 at 10:46 +0000, tip-bot for Ken Chen wrote:
> Commit-ID:  b0432d8f162c7d5d9537b4cb749d44076b76a783
> Gitweb:     http://git.kernel.org/tip/b0432d8f162c7d5d9537b4cb749d44076b76a783
> Author:     Ken Chen <kenchen@xxxxxxxxxx>
> AuthorDate: Thu, 7 Apr 2011 17:23:22 -0700
> Committer:  Ingo Molnar <mingo@xxxxxxx>
> CommitDate: Mon, 11 Apr 2011 11:08:54 +0200
> 
> sched: Fix sched-domain avg_load calculation
> 
> In function find_busiest_group(), the sched-domain avg_load isn't
> calculated at all if there is a group imbalance within the domain. This
> will cause erroneous imbalance calculation.
> 
> The reason is that calculate_imbalance() sees sds->avg_load = 0 and it
> will dump entire sds->max_load into imbalance variable, which is used
> later on to migrate entire load from busiest CPU to the puller CPU.
> 
> This has two really bad effect:
> 
> 1. stampede of task migration, and they won't be able to break out
>    of the bad state because of positive feedback loop: large load
>    delta -> heavier load migration -> larger imbalance and the cycle
>    goes on.
> 
> 2. severe imbalance in CPU queue depth.  This causes really long
>    scheduling latency blip which affects badly on application that
>    has tight latency requirement.
> 
> The fix is to have kernel calculate domain avg_load in both cases. This
> will ensure that imbalance calculation is always sensible and the target
> is usually half way between busiest and puller CPU.
> 
> Signed-off-by: Ken Chen <kenchen@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxx>
> Link: http://lkml.kernel.org/r/20110408002322.3A0D812217F@xxxxxxxxxxxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxx> 

This was caused by 866ab43ef (sched: Fix the group_imb logic) which is
only in .39-rc.
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