Re: [PATCH v10 04/11] sched: Make sched entity usage tracking scale-invariant

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 27/02/15 15:54, Vincent Guittot wrote:
> From: Morten Rasmussen <morten.rasmussen@arm.com>
> 
> Apply frequency scale-invariance correction factor to usage tracking.
> Each segment of the running_load_avg geometric series is now scaled by the

The same comment I sent out on [PATCH v10 07/11]:

The use of underscores in running_load_avg implies to me that this is a
data member of struct sched_avg or something similar. But there is no
running_load_avg in the current code. However, I can see that
sched_avg::*running_avg_sum* (and therefore
cfs_rq::*utilization_load_avg*) are frequency scale invariant.

-- Dietmar

> current frequency so the utilization_avg_contrib of each entity will be
> invariant with frequency scaling. As a result, utilization_load_avg which is
> the sum of utilization_avg_contrib, becomes invariant too. So the usage level
> that is returned by get_cpu_usage, stays relative to the max frequency as the
> cpu_capacity which is is compared against.
> Then, we want the keep the load tracking values in a 32bits type, which implies
> that the max value of {runnable|running}_avg_sum must be lower than
> 2^32/88761=48388 (88761 is the max weigth of a task). As LOAD_AVG_MAX = 47742,
> arch_scale_freq_capacity must return a value less than
> (48388/47742) << SCHED_CAPACITY_SHIFT = 1037 (SCHED_SCALE_CAPACITY = 1024).
> So we define the range to [0..SCHED_SCALE_CAPACITY] in order to avoid overflow.