Re: [PATCH v6 6/7] arm64: use activity monitors for frequency invariance

[Sudeep Holla <sudeep.holla@arm.com>]

On Thu, Mar 05, 2020 at 09:06:26AM +0000, Ionela Voinescu wrote:
> The Frequency Invariance Engine (FIE) is providing a frequency
> scaling correction factor that helps achieve more accurate
> load-tracking.
>
> So far, for arm and arm64 platforms, this scale factor has been
> obtained based on the ratio between the current frequency and the
> maximum supported frequency recorded by the cpufreq policy. The
> setting of this scale factor is triggered from cpufreq drivers by
> calling arch_set_freq_scale. The current frequency used in computation
> is the frequency requested by a governor, but it may not be the
> frequency that was implemented by the platform.
>
> This correction factor can also be obtained using a core counter and a
> constant counter to get information on the performance (frequency based
> only) obtained in a period of time. This will more accurately reflect
> the actual current frequency of the CPU, compared with the alternative
> implementation that reflects the request of a performance level from
> the OS.
>
> Therefore, implement arch_scale_freq_tick to use activity monitors, if
> present, for the computation of the frequency scale factor.
>
> The use of AMU counters depends on:
>  - CONFIG_ARM64_AMU_EXTN - depents on the AMU extension being present
>  - CONFIG_CPU_FREQ - the current frequency obtained using counter
>    information is divided by the maximum frequency obtained from the
>    cpufreq policy.
>
> While it is possible to have a combination of CPUs in the system with
> and without support for activity monitors, the use of counters for
> frequency invariance is only enabled for a CPU if all related CPUs
> (CPUs in the same frequency domain) support and have enabled the core
> and constant activity monitor counters. In this way, there is a clear
> separation between the policies for which arch_set_freq_scale (cpufreq
> based FIE) is used, and the policies for which arch_scale_freq_tick
> (counter based FIE) is used to set the frequency scale factor. For
> this purpose, a late_initcall_sync is registered to trigger validation
> work for policies that will enable or disable the use of AMU counters
> for frequency invariance. If CONFIG_CPU_FREQ is not defined, the use
> of counters is enabled on all CPUs only if all possible CPUs correctly
> support the necessary counters.
>
> Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
> Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Sudeep Holla <sudeep.holla@arm.com>
> ---
>  arch/arm64/include/asm/topology.h |   9 ++
>  arch/arm64/kernel/cpufeature.c    |   4 +
>  arch/arm64/kernel/topology.c      | 180 ++++++++++++++++++++++++++++++
>  drivers/base/arch_topology.c      |  12 ++
>  include/linux/arch_topology.h     |   2 +
>  5 files changed, 207 insertions(+)
>

[...]

> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 6119e11a9f95..8d63673c1689 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -21,6 +21,10 @@
>  #include <linux/sched.h>
>  #include <linux/smp.h>
>
> +__weak bool arch_freq_counters_available(struct cpumask *cpus)
> +{
> +	return false;
> +}
>  DEFINE_PER_CPU(unsigned long, freq_scale) = SCHED_CAPACITY_SCALE;
>
>  void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq,
> @@ -29,6 +33,14 @@ void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq,
>  	unsigned long scale;
>  	int i;
>
> +	/*
> +	 * If the use of counters for FIE is enabled, just return as we don't
> +	 * want to update the scale factor with information from CPUFREQ.
> +	 * Instead the scale factor will be updated from arch_scale_freq_tick.
> +	 */
> +	if (arch_freq_counters_available(cpus))
> +		return;
> +
>  	scale = (cur_freq << SCHED_CAPACITY_SHIFT) / max_freq;
>
>  	for_each_cpu(i, cpus)
> diff --git a/include/linux/arch_topology.h b/include/linux/arch_topology.h
> index 3015ecbb90b1..1ccdddb541a7 100644
> --- a/include/linux/arch_topology.h
> +++ b/include/linux/arch_topology.h
> @@ -33,6 +33,8 @@ unsigned long topology_get_freq_scale(int cpu)
>  	return per_cpu(freq_scale, cpu);
>  }
>
> +bool arch_freq_counters_available(struct cpumask *cpus);
> +
>  struct cpu_topology {
>  	int thread_id;
>  	int core_id;

Sorry for the delay. The arch_topology part looks fine to me. For that part:

Acked-by: Sudeep Holla <sudeep.holla@arm.com>

--
Regards,
Sudeep