Re: [PATCH v3 1/1] x86,sched: On AMD EPYC set freq_max = max_boost in schedutil invariant formula

From: Rafael J. Wysocki
Date: Wed Feb 03 2021 - 09:12:39 EST

On Wed, Feb 3, 2021 at 2:53 PM Giovanni Gherdovich <ggherdovich@xxxxxxx> wrote:
>

[cut]

> Fixes: 41ea667227ba ("x86, sched: Calculate frequency invariance for AMD systems")
> Fixes: 976df7e5730e ("x86, sched: Use midpoint of max_boost and max_P for frequency invariance on AMD EPYC")
> Reported-by: Michael Larabel <Michael@xxxxxxxxxxxx>
> Tested-by: Michael Larabel <Michael@xxxxxxxxxxxx>
> Signed-off-by: Giovanni Gherdovich <ggherdovich@xxxxxxx>
> ---
> drivers/cpufreq/acpi-cpufreq.c | 61 ++++++++++++++++++++++++++++++--
> drivers/cpufreq/cpufreq.c | 3 ++
> include/linux/cpufreq.h | 5 +++
> kernel/sched/cpufreq_schedutil.c | 8 +++--

I don't really think that it is necessary to modify schedutil to
address this issue.

> 4 files changed, 73 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
> index 1e4fbb002a31..a5facc6cad16 100644
> --- a/drivers/cpufreq/acpi-cpufreq.c
> +++ b/drivers/cpufreq/acpi-cpufreq.c
> @@ -26,6 +26,7 @@
> #include <linux/uaccess.h>
>
> #include <acpi/processor.h>
> +#include <acpi/cppc_acpi.h>
>
> #include <asm/msr.h>
> #include <asm/processor.h>
> @@ -628,11 +629,57 @@ static int acpi_cpufreq_blacklist(struct cpuinfo_x86 *c)
> }
> #endif
>
> +#ifdef CONFIG_ACPI_CPPC_LIB
> +static bool amd_max_boost(unsigned int max_freq,
> + unsigned int *max_boost)

Is there anything specific to AMD CPUs in this function?

> +{
> + struct cppc_perf_caps perf_caps;
> + u64 highest_perf, nominal_perf, perf_ratio;
> + int ret;
> +
> + ret = cppc_get_perf_caps(0, &perf_caps);
> + if (ret) {
> + pr_debug("Could not retrieve perf counters (%d)\n", ret);
> + return false;
> + }
> +
> + highest_perf = perf_caps.highest_perf;
> + nominal_perf = perf_caps.nominal_perf;
> +
> + if (!highest_perf || !nominal_perf) {
> + pr_debug("Could not retrieve highest or nominal performance\n");
> + return false;
> + }
> +
> + perf_ratio = div_u64(highest_perf * SCHED_CAPACITY_SCALE, nominal_perf);

Why do you use SCHED_CAPACITY_SCALE here? And why does this multiply
instead of shifting?

> + if (perf_ratio <= SCHED_CAPACITY_SCALE) {
> + pr_debug("Either perf_ratio is 0, or nominal >= highest performance\n");
> + return false;
> + }
> +
> + *max_boost = max_freq * perf_ratio >> SCHED_CAPACITY_SHIFT;

Is this assuming that max_freq corresponds to nominal_perf?

> + if (!*max_boost) {
> + pr_debug("max_boost seems to be zero\n");
> + return false;

So this function may just return the max_boost value with 0 meaning a failure.

> + }
> +
> + return true;
> +}
> +#else
> +static bool amd_max_boost(unsigned int max_freq,
> + unsigned int *max_boost)
> +{
> + return false;
> +}
> +#endif
> +
> static int acpi_cpufreq_cpu_init(struct cpufreq_policy *policy)
> {
> unsigned int i;
> unsigned int valid_states = 0;
> unsigned int cpu = policy->cpu;
> + unsigned int freq, max_freq = 0;
> + unsigned int max_boost;
> struct acpi_cpufreq_data *data;
> unsigned int result = 0;
> struct cpuinfo_x86 *c = &cpu_data(policy->cpu);
> @@ -779,15 +826,25 @@ static int acpi_cpufreq_cpu_init(struct cpufreq_policy *policy)
> freq_table[valid_states-1].frequency / 1000)
> continue;
>
> + freq = perf->states[i].core_frequency * 1000;
> freq_table[valid_states].driver_data = i;
> - freq_table[valid_states].frequency =
> - perf->states[i].core_frequency * 1000;
> + freq_table[valid_states].frequency = freq;
> +
> + if (freq > max_freq)
> + max_freq = freq;
> +
> valid_states++;
> }
> freq_table[valid_states].frequency = CPUFREQ_TABLE_END;
> policy->freq_table = freq_table;
> perf->state = 0;
>
> + if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD &&
> + amd_max_boost(max_freq, &max_boost)) {

AFAICS, the issue is not limited to AMD CPUs .

> + policy->cpuinfo.max_boost = max_boost;
> + static_branch_enable(&cpufreq_amd_max_boost);
> + }
> +
> switch (perf->control_register.space_id) {
> case ACPI_ADR_SPACE_SYSTEM_IO:
> /*
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index d0a3525ce27f..b96677f6b57e 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2721,6 +2721,9 @@ int cpufreq_boost_enabled(void)
> }
> EXPORT_SYMBOL_GPL(cpufreq_boost_enabled);
>
> +DEFINE_STATIC_KEY_FALSE(cpufreq_amd_max_boost);
> +EXPORT_SYMBOL_GPL(cpufreq_amd_max_boost);
> +
> /*********************************************************************
> * REGISTER / UNREGISTER CPUFREQ DRIVER *
> *********************************************************************/
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index 9c8b7437b6cd..341cac76d254 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -40,9 +40,14 @@ enum cpufreq_table_sorting {
> CPUFREQ_TABLE_SORTED_DESCENDING
> };
>
> +DECLARE_STATIC_KEY_FALSE(cpufreq_amd_max_boost);
> +
> +#define cpufreq_driver_has_max_boost() static_branch_unlikely(&cpufreq_amd_max_boost)
> +
> struct cpufreq_cpuinfo {
> unsigned int max_freq;
> unsigned int min_freq;
> + unsigned int max_boost;
>
> /* in 10^(-9) s = nanoseconds */
> unsigned int transition_latency;
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 6931f0cdeb80..541f3db3f576 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -159,8 +159,12 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> unsigned long util, unsigned long max)
> {
> struct cpufreq_policy *policy = sg_policy->policy;
> - unsigned int freq = arch_scale_freq_invariant() ?
> - policy->cpuinfo.max_freq : policy->cur;
> + unsigned int freq, max_freq;
> +
> + max_freq = cpufreq_driver_has_max_boost() ?
> + policy->cpuinfo.max_boost : policy->cpuinfo.max_freq;
> +
> + freq = arch_scale_freq_invariant() ? max_freq : policy->cur;
>
> freq = map_util_freq(util, freq, max);
>
> --