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

[Vincent Guittot]

On 19 April 2013 10:14, Mike Galbraith <efault@xxxxxx> wrote:
> On Fri, 2013-04-19 at 09:49 +0200, Vincent Guittot wrote:
>> 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.
>
> If it only scares fair class tasks away from the periodic rt load, that
> seems like a benefit to me, not a liability.  If we really really need

I'm not sure that such behavior that is only based on erroneous value,
is good one.

> perfect load numbers, fine, we have to eat some cycles, but when I look
> at it, it looks like one of those "Perfect is the enemy of good" things.

The target is not perfect number but good enough to be usable. The
systctl_migration_cost threshold is good for idle balancing but can
generates wrong load value

Vincent
>
> -Mike
>
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