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

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Date:	Thu, 3 Sep 2015 20:58:23 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>
Cc:	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

Hi Vincent,

On 02/09/15 10:31, Vincent Guittot wrote:
> Hi Morten,
> 
> On 14 August 2015 at 18:23, Morten Rasmussen <morten.rasmussen@....com> wrote:
>> Bring arch_scale_cpu_capacity() in line with the recent change of its
>> arch_scale_freq_capacity() sibling in commit dfbca41f3479 ("sched:
>> Optimize freq invariant accounting") from weak function to #define to
>> allow inlining of the function.
>>
>> While at it, remove the ARCH_CAPACITY sched_feature as well. With the
>> change to #define there isn't a straightforward way to allow runtime
>> switch between an arch implementation and the default implementation of
>> arch_scale_cpu_capacity() using sched_feature. The default was to use
>> the arch-specific implementation, but only the arm architecture provides
>> one and that is essentially equivalent to the default implementation.

[...]

> 
> So you change the way to declare arch_scale_cpu_capacity but i don't
> see the update of the arm arch which declare a
> arch_scale_cpu_capacity to reflect this change in your series.

We were reluctant to do this because this functionality makes only sense
for ARCH=arm big.Little systems w/ cortex-a{15|7} cores and only if the
clock-frequency property is set in the dts file. 

Are you planning to push for a 'struct cpu_efficiency/clock-frequency
property' solution for ARCH=arm64 as well?

I'm asking because for ARCH=arm64 systems today (JUNO, Hi6220) we use the
capacity value of the last entry of the capacity_state vector for the cores
(e.g. cortex-a{57|53). 
    
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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