Date:   Wed, 21 Jun 2017 17:38:28 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        Russell King <linux@....linux.org.uk>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Juri Lelli <juri.lelli@....com>,
        Vincent Guittot <vincent.guittot@...aor.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH 2/6] drivers base/arch_topology: frequency-invariant
 load-tracking support

On 20/06/17 07:17, Viresh Kumar wrote:
> On Thu, Jun 8, 2017 at 1:25 PM, Dietmar Eggemann
> <dietmar.eggemann@....com> wrote:
> 
>> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> 
>>  static int
>>  init_cpu_capacity_callback(struct notifier_block *nb,
>> @@ -185,6 +192,7 @@ init_cpu_capacity_callback(struct notifier_block *nb,
>>                                cpus_to_visit,
>>                                policy->related_cpus);
>>                 for_each_cpu(cpu, policy->related_cpus) {
>> +                       per_cpu(max_freq, cpu) = policy->cpuinfo.max_freq;
> 
> I am not sure about this but why shouldn't we use policy->max here ?
> As that is the
> max, we can set the frequency to right now.
> 

No, frequency invariance is defined by:

 current_freq(cpu) << SCHED_CAPACITY_SHIFT / max_supported_freq(cpu)

We don't want to scale against a value which might be restricted e.g.
by thermal capping.

>>                         if (cap_parsing_failed)
>>                                 continue;
>>                         raw_capacity[cpu] = topology_get_cpu_scale(NULL, cpu) *

[...]

>> +static int set_freq_scale_callback(struct notifier_block *nb,
>> +                                  unsigned long val,
>> +                                  void *data)
>> +{
>> +       struct cpufreq_freqs *freq = data;
>> +
>> +       switch (val) {
>> +       case CPUFREQ_PRECHANGE:
>> +               set_freq_scale(freq->cpu, freq->new);
> 
> Any specific reason on why are we doing this from PRECHANGE and
> not POSTCHANGE ? i.e. we are doing this before the frequency is
> really updated.

Not really. In case I get a CPUFREQ_POSTCHANGE all the time the
frequency actually changed I can switch to CPUFREQ_POSTCHANGE.

[...]

>> @@ -225,8 +265,14 @@ static int __init register_cpufreq_notifier(void)
>>
>>         cpumask_copy(cpus_to_visit, cpu_possible_mask);
>>
>> -       return cpufreq_register_notifier(&init_cpu_capacity_notifier,
>> -                                        CPUFREQ_POLICY_NOTIFIER);
>> +       ret = cpufreq_register_notifier(&init_cpu_capacity_notifier,
>> +                                       CPUFREQ_POLICY_NOTIFIER);
> 
> Wanted to make sure that we all understand the constraints this is going to add
> for the ARM64 platforms.
> 
> With the introduction of this transition notifier, we would not be able to use
> the fast-switch path in the schedutil governor. I am not sure if there are any
> ARM platforms that can actually use the fast-switch path in future or not
> though. The requirement of fast-switch path is that the freq can be changed
> without sleeping in the hot-path.

That's a good point. The cpufreq transition notifier based Frequency
Invariance Engine (FIE) can only work if none of the cpufreq policies
support fast frequency switching. 

What about we still enable cpufreq transition notifier based FIE for
systems where this is true. This will cover 100% of all arm/arm64
systems today.

In case one day we have a cpufreq driver which allows fast frequency
switching we would need a FIE based on something else than cpufreq
transition notifier. Maybe based on performance counters (something
similar to x86 APERF/MPERF) or cpufreq core could provide a function
which provides the avg frequency value.

I could make the current implementation more future-proof by only
using the notifier based FIE in case all policies use slow frequency
switching:

>From afe64b5c0606cad4304b77fc5cff819d3083a88d Mon Sep 17 00:00:00 2001
From: Dietmar Eggemann <dietmar.eggemann@....com>
Date: Wed, 21 Jun 2017 14:53:26 +0100
Subject: [PATCH] drivers base/arch_topology: enable cpufreq transistion
 notifier based FIE only for slow frequency switching

Fast frequency switching is incompatible with cpufreq transition
notifiers.

Enable the cpufreq transition notifier based Frequency Invariance Engine
(FIE) only in case there are no cpufreq policies able to use fast
frequency switching.

Currently there are no cpufreq drivers for arm/arm64 which support fast
frequency switching. In case such a driver will appear the FEI
topology_get_freq_scale() has to be extended to provide frequency
invariance based on something else than cpufreq transition notifiers,
e.g. performance counters.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
---
 drivers/base/arch_topology.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index c2539dc584d5..bd14c5e81f63 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -171,6 +171,7 @@ static bool cap_parsing_done;
 static void parsing_done_workfn(struct work_struct *work);
 static DECLARE_WORK(parsing_done_work, parsing_done_workfn);
 static DEFINE_PER_CPU(unsigned long, max_freq);
+static bool enable_freq_inv = true;

 static int
 init_cpu_capacity_callback(struct notifier_block *nb,
@@ -199,6 +200,8 @@ init_cpu_capacity_callback(struct notifier_block *nb,
                                            policy->cpuinfo.max_freq / 1000UL;
                        capacity_scale = max(raw_capacity[cpu], capacity_scale);
                }
+               if (policy->fast_switch_possible)
+                       enable_freq_inv = false;
                if (cpumask_empty(cpus_to_visit)) {
                        if (!cap_parsing_failed) {
                                topology_normalize_cpu_scale();
@@ -268,21 +271,23 @@ static int __init register_cpufreq_notifier(void)
        ret = cpufreq_register_notifier(&init_cpu_capacity_notifier,
                                        CPUFREQ_POLICY_NOTIFIER);

-       if (ret) {
+       if (ret)
                free_cpumask_var(cpus_to_visit);
-               return ret;
-       }

-       return cpufreq_register_notifier(&set_freq_scale_notifier,
-                                        CPUFREQ_TRANSITION_NOTIFIER);
+       return ret;
 }
 core_initcall(register_cpufreq_notifier);

 static void parsing_done_workfn(struct work_struct *work)
 {
+
        free_cpumask_var(cpus_to_visit);
        cpufreq_unregister_notifier(&init_cpu_capacity_notifier,
                                         CPUFREQ_POLICY_NOTIFIER);
+
+       if (enable_freq_inv)
+               cpufreq_register_notifier(&set_freq_scale_notifier,
+                                         CPUFREQ_TRANSITION_NOTIFIER);
 }

 #else
-- 
2.11.0

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