Re: [RFCv5, 01/46] arm: Frequency invariant scheduler load-tracking support

[Leo Yan <leo.yan@linaro.org>]

On Wed, Jul 22, 2015 at 02:31:04PM +0100, Morten Rasmussen wrote:
> On Tue, Jul 21, 2015 at 11:41:45PM +0800, Leo Yan wrote:
> > Hi Morten,
> > 
> > On Tue, Jul 07, 2015 at 07:23:44PM +0100, Morten Rasmussen wrote:
> > > From: Morten Rasmussen <Morten.Rasmussen@arm.com>
> > > 
> > > Implements arch-specific function to provide the scheduler with a
> > > frequency scaling correction factor for more accurate load-tracking.
> > > The factor is:
> > > 
> > > 	current_freq(cpu) << SCHED_CAPACITY_SHIFT / max_freq(cpu)
> > > 
> > > This implementation only provides frequency invariance. No cpu
> > > invariance yet.
> > > 
> > > Cc: Russell King <linux@arm.linux.org.uk>
> > > 
> > > Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
> > > 
> > > ---
> > > arch/arm/include/asm/topology.h |  7 +++++
> > >  arch/arm/kernel/smp.c           | 57 +++++++++++++++++++++++++++++++++++++++--
> > >  arch/arm/kernel/topology.c      | 17 ++++++++++++
> > >  3 files changed, 79 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/arm/include/asm/topology.h b/arch/arm/include/asm/topology.h
> > > index 370f7a7..c31096f 100644
> > > --- a/arch/arm/include/asm/topology.h
> > > +++ b/arch/arm/include/asm/topology.h
> > > @@ -24,6 +24,13 @@ void init_cpu_topology(void);
> > >  void store_cpu_topology(unsigned int cpuid);
> > >  const struct cpumask *cpu_coregroup_mask(int cpu);
> > >  
> > > +#define arch_scale_freq_capacity arm_arch_scale_freq_capacity
> > > +struct sched_domain;
> > > +extern
> > > +unsigned long arm_arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
> > > +
> > > +DECLARE_PER_CPU(atomic_long_t, cpu_freq_capacity);
> > > +
> > >  #else
> > >  
> > >  static inline void init_cpu_topology(void) { }
> > > diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> > > index cca5b87..a32539c 100644
> > > --- a/arch/arm/kernel/smp.c
> > > +++ b/arch/arm/kernel/smp.c
> > > @@ -677,12 +677,34 @@ static DEFINE_PER_CPU(unsigned long, l_p_j_ref);
> > >  static DEFINE_PER_CPU(unsigned long, l_p_j_ref_freq);
> > >  static unsigned long global_l_p_j_ref;
> > >  static unsigned long global_l_p_j_ref_freq;
> > > +static DEFINE_PER_CPU(atomic_long_t, cpu_max_freq);
> > > +DEFINE_PER_CPU(atomic_long_t, cpu_freq_capacity);
> > > +
> > > +/*
> > > + * Scheduler load-tracking scale-invariance
> > > + *
> > > + * Provides the scheduler with a scale-invariance correction factor that
> > > + * compensates for frequency scaling through arch_scale_freq_capacity()
> > > + * (implemented in topology.c).
> > > + */
> > > +static inline
> > > +void scale_freq_capacity(int cpu, unsigned long curr, unsigned long max)
> > > +{
> > > +	unsigned long capacity;
> > > +
> > > +	if (!max)
> > > +		return;
> > > +
> > > +	capacity = (curr << SCHED_CAPACITY_SHIFT) / max;
> > > +	atomic_long_set(&per_cpu(cpu_freq_capacity, cpu), capacity);
> > > +}
> > >  
> > >  static int cpufreq_callback(struct notifier_block *nb,
> > >  					unsigned long val, void *data)
> > >  {
> > >  	struct cpufreq_freqs *freq = data;
> > >  	int cpu = freq->cpu;
> > > +	unsigned long max = atomic_long_read(&per_cpu(cpu_max_freq, cpu));
> > >  
> > >  	if (freq->flags & CPUFREQ_CONST_LOOPS)
> > >  		return NOTIFY_OK;
> > > @@ -707,6 +729,10 @@ static int cpufreq_callback(struct notifier_block *nb,
> > >  					per_cpu(l_p_j_ref_freq, cpu),
> > >  					freq->new);
> > >  	}
> > > +
> > > +	if (val == CPUFREQ_PRECHANGE)
> > > +		scale_freq_capacity(cpu, freq->new, max);
> > > +
> > >  	return NOTIFY_OK;
> > >  }
> > >  
> > > @@ -714,11 +740,38 @@ static struct notifier_block cpufreq_notifier = {
> > >  	.notifier_call  = cpufreq_callback,
> > >  };
> > >  
> > > +static int cpufreq_policy_callback(struct notifier_block *nb,
> > > +						unsigned long val, void *data)
> > > +{
> > > +	struct cpufreq_policy *policy = data;
> > > +	int i;
> > > +
> > > +	if (val != CPUFREQ_NOTIFY)
> > > +		return NOTIFY_OK;
> > > +
> > > +	for_each_cpu(i, policy->cpus) {
> > > +		scale_freq_capacity(i, policy->cur, policy->max);
> > > +		atomic_long_set(&per_cpu(cpu_max_freq, i), policy->max);
> > > +	}
> > > +
> > > +	return NOTIFY_OK;
> > > +}
> > > +
> > > +static struct notifier_block cpufreq_policy_notifier = {
> > > +	.notifier_call	= cpufreq_policy_callback,
> > > +};
> > > +
> > >  static int __init register_cpufreq_notifier(void)
> > >  {
> > > -	return cpufreq_register_notifier(&cpufreq_notifier,
> > > +	int ret;
> > > +
> > > +	ret = cpufreq_register_notifier(&cpufreq_notifier,
> > >  						CPUFREQ_TRANSITION_NOTIFIER);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	return cpufreq_register_notifier(&cpufreq_policy_notifier,
> > > +						CPUFREQ_POLICY_NOTIFIER);
> > >  }
> > >  core_initcall(register_cpufreq_notifier);
> > 
> > For "cpu_freq_capacity" structure, could move it into driver/cpufreq
> > so that it can be shared by all architectures? Otherwise, every
> > architecture's smp.c need register notifier for themselves.
> 
> We could, but I put it in arch/arm/* as not all architectures might want
> this notifier. The frequency scaling factor could be provided based on
> architecture specific performance counters instead. AFAIK, the Intel
> p-state driver does not even fire the notifiers so the notifier
> solution would be redundant code for those platforms.

When i tried to enable EAS on Hikey, i found it's absent related code
for arm64; actually this code section can also be reused by arm64,
so just brought up this question.

Just now roughly went through the driver
"drivers/cpufreq/intel_pstate.c"; that's true it has different
implementation comparing to usual ARM SoCs. So i'd like to ask this
question with another way: should cpufreq framework provides helper
functions for getting related cpu frequency scaling info? If the
architecture has specific performance counters then it can ignore
these helper functions.

> That said, the above solution is not handling changes to policy->max
> very well. Basically, we don't inform the scheduler if it has changed
> which means that the OPP represented by "100%" might change. We need
> cpufreq to keep track of the true max frequency when policy->max is
> changed to work out the correct scaling factor instead of having it
> relative to policy->max.

i'm not sure understand correctly here. For example, when thermal
framework limits the cpu frequency, it will update the value for
policy->max, so scheduler will get the correct scaling factor, right?
So i don't know what's the issue at here.

Further more, i noticed in the later patches for
arch_scale_cpu_capacity(); the cpu capacity is calculated by the
property passed by DT, so it's a static value. In some cases, system
may constraint the maximum frequency for CPUs, so in this case, will
scheduler get misknowledge from arch_scale_cpu_capacity after system
has imposed constraint for maximum frequency?

Sorry if these questions have been discussed before :)

Thanks,
Leo Yan