Re: [PATCH v5 11/12] sched: replace capacity_factor by utilization

[Vincent Guittot]

On 15 September 2014 13:42, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Sun, Sep 14, 2014 at 09:41:56PM +0200, Peter Zijlstra wrote:
>> On Thu, Sep 11, 2014 at 07:26:48PM +0200, Vincent Guittot wrote:
>> > On 11 September 2014 18:15, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
>> > > I'm confused about the utilization vs capacity_orig. I see how we should
>> >
>> > 1st point is that I should compare utilization vs capacity and not
>> > capacity_orig.
>> > I should have replaced capacity_orig by capacity in the functions
>> > above when i move the utilization statistic from
>> > rq->avg.runnable_avg_sum to cfs.usage_load_avg.
>> > rq->avg.runnable_avg_sum was measuring all activity on the cpu whereas
>> > cfs.usage_load_avg integrates only cfs tasks
>> >
>> > With this change, we don't need sgs->group_capacity_orig anymore but
>> > only sgs->group_capacity. So sgs->group_capacity_orig can be removed
>> > as it's no more used in the code as sg_capacity_factor has been
>> > removed
>>
>> Yes, but.. so I suppose we need to add DVFS accounting and remove
>> cpufreq from the capacity thing. Otherwise I don't see it make sense.
>
> OK, I've reconsidered _again_, I still don't get it.
>
> So fundamentally I think its wrong to scale with the capacity; it just
> doesn't make any sense. Consider big.little stuff, their CPUs are
> inherently asymmetric in capacity, but that doesn't matter one whit for
> utilization numbers. If a core is fully consumed its fully consumed, no
> matter how much work it can or can not do.
>
>
> So the only thing that needs correcting is the fact that these
> statistics are based on clock_task and some of that time can end up in
> other scheduling classes, at which point we'll never get 100% even
> though we're 'saturated'. But correcting for that using capacity doesn't
> 'work'.

I'm not sure to catch your last point because the capacity is the only
figures that take into account the "time" consumed by other classes.
Have you got in mind another way to take into account the other
classes ?

So we have cpu_capacity that is the capacity that can be currently
used by cfs class
We have cfs.usage_load_avg that is the sum of running time of cfs
tasks on the CPU and reflect the % of usage of this CPU by CFS tasks
We have to use the same metrics to compare available capacity for CFS
and current cfs usage

Now we have to use the same unit so we can either weight the
cpu_capacity_orig with the cfs.usage_load_avg and compare it with
cpu_capacity
or with divide cpu_capacity by cpu_capacity_orig and scale it into the
SCHED_LOAD_SCALE range. Is It what you are proposing ?

Vincent
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