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

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Message-ID: <3c3f6d32-3560-2c54-beae-36f53d2572e7@arm.com>
Date:   Fri, 14 May 2021 18:16:50 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Ionela Voinescu <ionela.voinescu@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/3] arch_topology: obtain cpu capacity using information
 from CPPC

On 14/05/2021 11:53, Ionela Voinescu wrote:

[...]

> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index c1179edc0f3b..f710d64f125b 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -291,6 +291,45 @@ 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>

init_cpu_capacity_cppc() shares a lot of functionality with the existing
DT/CPUfreq-based approach (topology_parse_cpu_capacity(),
register_cpufreq_notifier(), init_cpu_capacity_callback()). It looks
like that the different ways of invocation (two steps per cpu vs. one
step for all cpus) makes it hard to restructure the code to create more
common bits.

> +void init_cpu_capacity_cppc(void)
> +{
> +	struct cppc_perf_caps perf_caps;
> +	int cpu;
> +
> +	if (likely(acpi_disabled || !acpi_cpc_valid()))

likely(acpi_disabled) ?

> +		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)) {
> +			raw_capacity[cpu] = perf_caps.highest_perf;
> +			pr_debug("%s: CPU%d cpu_capacity=%u (raw).\n",
> +				 __func__, cpu, raw_capacity[cpu]);

There is quite a variety in the layout of the pr_xxx() log messages in
this file. Originally the 'cpu_capacity:' was used to indicate that this
log is from drivers/base/arch_topology.c. Now the GCC __func__
identifier is used. Maybe this can be aligned better? Especially since
the functionality used in the existing DT-driven and now in the new
CPPC-driven functionality is quite similar. Debugging is so much easier
with consistent log strings.

> +		} else {
> +			pr_err("%s: CPU%d missing highest performance.\n",
> +				 __func__, cpu);
> +			pr_err("%s: fallback to 1024 for all CPUs\n",
> +				 __func__);
> +			goto exit;
> +		}
> +	}
> +
> +	topology_normalize_cpu_scale();
> +	schedule_work(&update_topology_flags_work);
> +	pr_debug("%s: cpu_capacity initialization done\n", __func__);
> +
> +exit:
> +	free_raw_capacity();
> +}
> +#endif

In case a system has CONFIG_ACPI_CPPC_LIB what does this mean for the
DT-based approach via `register_cpufreq_notifier()`?

Looks like we rely on:

376 static int __init register_cpufreq_notifier(void)
...
385         if (!acpi_disabled || ...)
386                 return -EINVAL;

to disable the CPUfreq part of the DT/CPUfreq-based approach on an ACPI
system.

[...]