Re: [PATCH Resend v6] sched: fix wrong rq's runnable_avg update withrt tasks

[Vincent Guittot]

On 19 April 2013 06:30, Mike Galbraith <efault@xxxxxx> wrote:
> On Thu, 2013-04-18 at 18:34 +0200, Vincent Guittot wrote:
>> The current update of the rq's load can be erroneous when RT tasks are
>> involved
>>
>> The update of the load of a rq that becomes idle, is done only if the avg_idle
>> is less than sysctl_sched_migration_cost. If RT tasks and short idle duration
>> alternate, the runnable_avg will not be updated correctly and the time will be
>> accounted as idle time when a CFS task wakes up.
>>
>> A new idle_enter function is called when the next task is the idle function
>> so the elapsed time will be accounted as run time in the load of the rq,
>> whatever the average idle time is. The function update_rq_runnable_avg is
>> removed from idle_balance.
>>
>> When a RT task is scheduled on an idle CPU, the update of the rq's load is
>> not done when the rq exit idle state because CFS's functions are not
>> called. Then, the idle_balance, which is called just before entering the
>> idle function, updates the rq's load and makes the assumption that the
>> elapsed time since the last update, was only running time.
>>
>> As a consequence, the rq's load of a CPU that only runs a periodic RT task,
>> is close to LOAD_AVG_MAX whatever the running duration of the RT task is.
>
> Why do we care what rq's load says, if the only thing running is a
> periodic RT task?  I _think_ I recall that stuff being put under the

cfs scheduler will use a wrong rq load the next time it wants to schedule a task

> throttle specifically to not waste cycles doing that on every
> microscopic idle.

yes but this lead to the wrong computation of runnable_avg_sum. To be
more precise, we only need to call __update_entity_runnable_avg,
__update_tg_runnable_avg is not mandatory in this step.

>
> Seems to me when scheduling an rt task, you want to do as little other
> than switching to/from the rt task as possible.  I don't let rt tasks do
> idle balancing either, their job isn't to balance fair class on the way
> out the door, it's to get off/onto the cpu ASAP, and do rt work.

I agree but the patch is not about balancing fair task but keep
coherent runnable value

Vincent
>
> -Mike
>
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