linux-kernel - Re: [PATCH v2 08/11] sched: get CPU's activity statistic

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Date:	Wed, 28 May 2014 09:01:04 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v2 08/11] sched: get CPU's activity statistic

On 27 May 2014 19:32, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, May 23, 2014 at 05:53:02PM +0200, Vincent Guittot wrote:
>> Monitor the activity level of each group of each sched_domain level. The
>> activity is the amount of cpu_power that is currently used on a CPU or group
>> of CPUs. We use the runnable_avg_sum and _period to evaluate this activity
>> level. In the special use case where the CPU is fully loaded by more than 1
>> task, the activity level is set above the cpu_power in order to reflect the
>> overload of the CPU
>>
>
>> +static int get_cpu_activity(int cpu)
>> +{
>> +     struct rq *rq = cpu_rq(cpu);
>> +     u32 sum = rq->avg.runnable_avg_sum;
>> +     u32 period = rq->avg.runnable_avg_period;
>> +
>> +     if (sum >= period)
>> +             return power_orig_of(cpu) + rq->nr_running - 1;
>> +
>> +     return (sum * power_orig_of(cpu)) / period;
>> +}
>
> While I appreciate the need to signify the overload situation, I don't
> think adding nr_running makes sense. The amount of tasks has no bearing
> on the amount of overload.

I agree that it's not the best way to evaluate overload but it's the
only simple one that is available without additional computation. I'm
going to try to find a better metric or come back the solution which
only adds +1  and compute the overload somewhere else

>
> Also, and note I've not yet seen the use, it strikes me as odd to use
> the orig power here. I would've thought the current capacity (not the
> max capacity) is relevant to balancing.

activity also measures the non cfs tasks activity whereas current
capacity removes the capacity used by non cfs tasks. So as long as
arch_scale_cpu_freq is not used (which is the case for now), original
capacity is ok but we might need a 3rd metric so we would have:
original capacity (max capacity that can provide a CPU)
usable capacity (new value that reflects current capacity available
for any kind of processing rt tasks, irq, cfs tasks)
current capacity (capacity available for cfs tasks)

Vincent
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