linux-kernel - Re: [PATCH 2/6] sched/fair: Convert arch_scale_cpu

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Date:	Fri, 11 Sep 2015 14:21:32 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Morten Rasmussen <Morten.Rasmussen@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Yuyang Du <yuyang.du@...el.com>,
	Michael Turquette <mturquette@...libre.com>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	Sai Charan Gurrappadi <sgurrappadi@...dia.com>,
	"pang.xunlei@....com.cn" <pang.xunlei@....com.cn>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/6] sched/fair: Convert arch_scale_cpu_capacity() from
 weak function to #define

On 04/09/15 08:26, Vincent Guittot wrote:
> On 3 September 2015 at 21:58, Dietmar Eggemann <dietmar.eggemann@....com> wrote:

[...]

> So, with the patch below that updates the arm definition of
> arch_scale_cpu_capacity, you can add my Acked-by: Vincent Guittot
> <vincent.guittot@...aro.org> on this patch and the additional one
> below

My tests on ARM TC2 (only the 2 A15's) show that the influence of the
non-default arch_scale_cpu_capacity function in
arch/arm/kernel/topology.c and with it the extra function call to it
from __update_load_avg() has no measurable influence on performance:

perf stat --null --repeat 10 -- perf bench sched messaging --g 50 -l 200

- default arch_scale_cpu_capacity function [kernel/sched/sched.h]

16.006976251 seconds time elapsed ( +- 0.29% )
16.063814914 seconds time elapsed ( +- 0.37% )
16.088199252 seconds time elapsed ( +- 0.39% )

- arch_scale_cpu_capacity function [arch/arm/kernel/topology.c]

15.945975308 seconds time elapsed ( +- 0.10% )
16.131203074 seconds time elapsed ( +- 0.21% )
16.108302562 seconds time elapsed ( +- 0.41% )

If I force the function to be inline, the result is slightly worse:

- arch_scale_cpu_capacity function [arch/arm/include/asm/topology.h]

16.122545216 seconds time elapsed ( +- 0.09% )
16.285819258 seconds time elapsed ( +- 0.35% )
16.157454024 seconds time elapsed ( +- 0.15% )

So I think we can connect the arch_scale_cpu_capacity function
[arch/arm/kernel/topology.c] to the CFS scheduler for ARCH=arm so that
people get cpu scale different to 1024 on arm big.little machines w/
A15/A7 in case the specify clock-frequency properties in their dtb file.

Can we still have your 'Acked-by' for this patch and 3/6 even though we
now scale weight (by frequency) and scale_freq (by cpu) instead of the
time related values (delta_w, contrib, delta)?

Thanks,

-- Dietmar

> 
> Regards,
> Vincent
> 
>>
>> To connect the cpu invariant engine (scale_cpu_capacity()
>> [arch/arm/kernel/topology.c]) with the scheduler, something like this is
>> missing:
>>
>> diff --git a/arch/arm/include/asm/topology.h b/arch/arm/include/asm/topology.h
>> index 370f7a732900..17c6b3243196 100644
>> --- a/arch/arm/include/asm/topology.h
>> +++ b/arch/arm/include/asm/topology.h
>> @@ -24,6 +24,10 @@ void init_cpu_topology(void);
>>  void store_cpu_topology(unsigned int cpuid);
>>  const struct cpumask *cpu_coregroup_mask(int cpu);
>>
>> +#define arch_scale_cpu_capacity scale_cpu_capacity
>> +struct sched_domain;
>> +extern unsigned long scale_cpu_capacity(struct sched_domain *sd, int cpu);
>> +
>>  #else
>>
>>  static inline void init_cpu_topology(void) { }
>> diff --git a/arch/arm/kernel/topology.c b/arch/arm/kernel/topology.c
>> index 08b7847bf912..907e0d2d9b82 100644
>> --- a/arch/arm/kernel/topology.c
>> +++ b/arch/arm/kernel/topology.c
>> @@ -42,7 +42,7 @@
>>   */
>>  static DEFINE_PER_CPU(unsigned long, cpu_scale);
>>
>> -unsigned long arch_scale_cpu_capacity(struct sched_domain *sd, int cpu)
>> +unsigned long scale_cpu_capacity(struct sched_domain *sd, int cpu)
>>  {
>>         return per_cpu(cpu_scale, cpu);
>>  }
>> @@ -166,7 +166,7 @@ static void update_cpu_capacity(unsigned int cpu)
>>         set_capacity_scale(cpu, cpu_capacity(cpu) / middle_capacity);
>>
>>         pr_info("CPU%u: update cpu_capacity %lu\n",
>> -               cpu, arch_scale_cpu_capacity(NULL, cpu));
>> +               cpu, scale_cpu_capacity(NULL, cpu));
>>  }
>>
>> -- Dietmar
>>
>> [...]

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