linux-kernel - Re: [RFCv3 PATCH 12/48] sched: Make usage tracking cpu scale-invariant

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Date:	Mon, 23 Mar 2015 19:19:41 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>
CC:	"mingo@...hat.com" <mingo@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"yuyang.du@...el.com" <yuyang.du@...el.com>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"mturquette@...aro.org" <mturquette@...aro.org>,
	"nico@...aro.org" <nico@...aro.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFCv3 PATCH 12/48] sched: Make usage tracking cpu scale-invariant

On 23/03/15 14:46, Peter Zijlstra wrote:
> On Wed, Feb 04, 2015 at 06:30:49PM +0000, Morten Rasmussen wrote:
>> From: Dietmar Eggemann <dietmar.eggemann@....com>
>>
>> Besides the existing frequency scale-invariance correction factor, apply
>> cpu scale-invariance correction factor to usage tracking.
>>
>> Cpu scale-invariance takes cpu performance deviations due to
>> micro-architectural differences (i.e. instructions per seconds) between
>> cpus in HMP systems (e.g. big.LITTLE) and differences in the frequency
>> value of the highest OPP between cpus in SMP systems into consideration.
>>
>> Each segment of the sched_avg::running_avg_sum geometric series is now
>> scaled by the cpu performance factor too so the
>> sched_avg::utilization_avg_contrib of each entity will be invariant from
>> the particular cpu of the HMP/SMP system it is gathered on.
>>
>> So the usage level that is returned by get_cpu_usage stays relative to
>> the max cpu performance of the system.
>
>> @@ -2547,6 +2549,10 @@ static __always_inline int __update_entity_runnable_avg(u64 now, int cpu,
>>
>>   		if (runnable)
>>   			sa->runnable_avg_sum += scaled_delta_w;
>> +
>> +		scaled_delta_w *= scale_cpu;
>> +		scaled_delta_w >>= SCHED_CAPACITY_SHIFT;
>> +
>>   		if (running)
>>   			sa->running_avg_sum += scaled_delta_w;
>>   		sa->avg_period += delta_w;
>
> Maybe help remind me why we want this asymmetry between runnable and
> running in terms of scaling?

In the previous patch-set https://lkml.org/lkml/2014/12/2/332 we 
cpu-scaled both (sched_avg::runnable_avg_sum (load) and 
sched_avg::running_avg_sum (utilization)) but during the review Vincent 
pointed out that a cpu-scaled invariant load signal messes up 
load-balancing based on s[dg]_lb_stats::avg_load in overload scenarios.

avg_load = load/capacity and load can't be simply replaced here by 
'cpu-scale invariant load' (which is load*capacity).

> The above talks about why we want running scaled with the cpu metric,
> but it forgets to tell me why we do not want to scale runnable.

Yes, I will add the missing explanation to this patch.

> (even if I were to have a vague recollection it seems like a good thing
> to write down someplace ;-).

Definitely true.

Back in December last year we talked about adding the now missing 
cpu-scale invariant load signal to the end (which should contain more 
experimental bits) of the patch-set. I guess we haven't done this simply 
because of the missing modifications around s[dg]_lb_stats::avg_load 
which would be then needed.

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