Re: [patch v3 6/8] sched: consider runnable load average in move_tasks

From: Vincent Guittot
Date: Tue Apr 09 2013 - 07:56:48 EST

On 9 April 2013 12:38, Alex Shi <alex.shi@xxxxxxxxx> wrote:
> On 04/09/2013 04:58 PM, Vincent Guittot wrote:
>>>> >> How do you ensure that runnable_avg_period and runnable_avg_sum are
>>>> >> coherent ? an update of the statistic can occur in the middle of your
>>>> >> sequence.
>>> >
>>> > Thanks for your question, Vincent!
>>> > the runnable_avg_period and runnable_avg_sum, only updated in
>>> > __update_entity_runnable_avg().
>>> > Yes, I didn't see some locks to ensure the coherent of them. but they
>>> > are updated closely, and it is not big deal even a little incorrect to
>>> > their value. These data are collected periodically, don't need very very
>>> > precise at every time.
>>> > Am I right? :)
>> The problem mainly appears during starting phase (the 1st 345ms) when
>> runnable_avg_period has not reached the max value yet so you can have
>> avg.runnable_avg_sum greater than avg.runnable_avg_period. In a worst
>> case, runnable_avg_sum could be twice runnable_avg_period
> Oh, That's a serious problem. Do you catch it in real word or in code?

I haven't trace that shows this issue but nothing prevent an update to
occur while you get values so you can have a mix of old and new

> Could you explain more for details?

Both fields of a new task increase simultaneously but if you get the
old value for runnable_avg_period and the new one for
runnable_avg_sum, runnable_avg_sum will be greater than
runnable_avg_period during this starting phase.

The worst case appears 2ms after the creation of the task,
runnable_avg_period and runnable_avg_sum should go from 1024 to 2046.
So the task_h_load_avg will be 199% of task_h_load If you have
runnable_avg_period with 1024 and runnable_avg_sum with 2046.

A simple solution is to check that runnable_avg_period is always
greater or equal to runnable_avg_sum like i have done in my packing
small tasks patches

> --
> Thanks
> Alex
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