Re: [PATCH v3 2/3] arch_topology: obtain cpu capacity using information from CPPC

Yicong Yang
2022-03-09T10:21:36Z

Hi Ionela,

On 2022/3/3 2:09, Ionela Voinescu wrote:
> Define topology_init_cpu_capacity_cppc() to use highest performance
> values from _CPC objects to obtain and set maximum capacity information
> for each CPU. acpi_cppc_processor_probe() is a good point at which to
> trigger the initialization of CPU (u-arch) capacity values, as at this
> point the highest performance values can be obtained from each CPU's
> _CPC objects. Architectures can therefore use this functionality
> through arch_init_invariance_cppc().
>
> The performance scale used by CPPC is a unified scale for all CPUs in
> the system. Therefore, by obtaining the raw highest performance values
> from the _CPC objects, and normalizing them on the [0, 1024] capacity
> scale, used by the task scheduler, we obtain the CPU capacity of each
> CPU.
>

So we're going to use highest performance rather than nominal performance,
and I checked the discussion in v2 [1]. Maybe we should also document this
in sched-capacity.rst that where scheduler get the capacity from on ACPI
based system? Currently we only have DT part but after this patch it's
also supported on ACPI based system.

Out of curiosity, since we have raw capacity now on ACPI system, seems we
are able to scale the capacity with freq_factor now? looked into
register_cpufreq_notifier().

[1] https://lore.kernel.org/lkml/Yh5OAsYVBWWko+CH@arm.com/

Thanks,
Yicong

> While an ACPI Notify(0x85) could alert about a change in the highest
> performance value, which should in turn retrigger the CPU capacity
> computations, this notification is not currently handled by the ACPI
> processor driver. When supported, a call to arch_init_invariance_cppc()
> would perform the update.
>
> Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
> Tested-by: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Sudeep Holla <sudeep.holla@arm.com>
> ---
> drivers/base/arch_topology.c | 40 +++++++++++++++++++++++++++++++++++
> include/linux/arch_topology.h | 4 ++++
> 2 files changed, 44 insertions(+)
>
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 976154140f0b..ad2d95920ad1 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -339,6 +339,46 @@ bool __init topology_parse_cpu_capacity(struct device_node *cpu_node, int cpu)
> return !ret;
> }
>
> +#ifdef CONFIG_ACPI_CPPC_LIB
> +#include <acpi/cppc_acpi.h>
> +
> +void topology_init_cpu_capacity_cppc(void)
> +{
> + struct cppc_perf_caps perf_caps;
> + int cpu;
> +
> + if (likely(acpi_disabled || !acpi_cpc_valid()))
> + return;
> +
> + raw_capacity = kcalloc(num_possible_cpus(), sizeof(*raw_capacity),
> + GFP_KERNEL);
> + if (!raw_capacity)
> + return;
> +
> + for_each_possible_cpu(cpu) {
> + if (!cppc_get_perf_caps(cpu, &perf_caps) &&
> + (perf_caps.highest_perf >= perf_caps.nominal_perf) &&
> + (perf_caps.highest_perf >= perf_caps.lowest_perf)) {
> + raw_capacity[cpu] = perf_caps.highest_perf;
> + pr_debug("cpu_capacity: CPU%d cpu_capacity=%u (raw).\n",
> + cpu, raw_capacity[cpu]);
> + continue;
> + }
> +
> + pr_err("cpu_capacity: CPU%d missing/invalid highest performance.\n", cpu);
> + pr_err("cpu_capacity: partial information: fallback to 1024 for all CPUs\n");
> + goto exit;
> + }
> +
> + topology_normalize_cpu_scale();
> + schedule_work(&update_topology_flags_work);
> + pr_debug("cpu_capacity: cpu_capacity initialization done\n");
> +
> +exit:
> + free_raw_capacity();
> +}
> +#endif
> +
> #ifdef CONFIG_CPU_FREQ
> static cpumask_var_t cpus_to_visit;
> static void parsing_done_workfn(struct work_struct *work);
> diff --git a/include/linux/arch_topology.h b/include/linux/arch_topology.h
> index cce6136b300a..58cbe18d825c 100644
> --- a/include/linux/arch_topology.h
> +++ b/include/linux/arch_topology.h
> @@ -11,6 +11,10 @@
> void topology_normalize_cpu_scale(void);
> int topology_update_cpu_topology(void);
>
> +#ifdef CONFIG_ACPI_CPPC_LIB
> +void topology_init_cpu_capacity_cppc(void);
> +#endif
> +
> struct device_node;
> bool topology_parse_cpu_capacity(struct device_node *cpu_node, int cpu);
>
>