Re: [PATCH v9 4/5] arm64: Provide an AMU-based version of arch_freq_get_on_cpu

Sumit Gupta
2025-01-29T11:15:47Z

On 21/01/25 14:14, Beata Michalska wrote:
>
>
> With the Frequency Invariance Engine (FIE) being already wired up with
> sched tick and making use of relevant (core counter and constant
> counter) AMU counters, getting the average frequency for a given CPU,
> can be achieved by utilizing the frequency scale factor which reflects
> an average CPU frequency for the last tick period length.
>
> The solution is partially based on APERF/MPERF implementation of
> arch_freq_get_on_cpu.
>
> Suggested-by: Ionela Voinescu <ionela.voinescu@arm.com>
> Signed-off-by: Beata Michalska <beata.michalska@arm.com>

Reviewed-by: Sumit Gupta <sumitg@nvidia.com>

> ---
> arch/arm64/kernel/topology.c | 109 +++++++++++++++++++++++++++++++----
> 1 file changed, 99 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index cb180684d10d..5f5738b174c7 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -17,6 +17,7 @@
> #include <linux/cpufreq.h>
> #include <linux/init.h>
> #include <linux/percpu.h>
> +#include <linux/sched/isolation.h>
>
> #include <asm/cpu.h>
> #include <asm/cputype.h>
> @@ -88,18 +89,28 @@ int __init parse_acpi_topology(void)
> * initialized.
> */
> static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, arch_max_freq_scale) = 1UL << (2 * SCHED_CAPACITY_SHIFT);
> -static DEFINE_PER_CPU(u64, arch_const_cycles_prev);
> -static DEFINE_PER_CPU(u64, arch_core_cycles_prev);
> static cpumask_var_t amu_fie_cpus;
>
> +struct amu_cntr_sample {
> + u64 arch_const_cycles_prev;
> + u64 arch_core_cycles_prev;
> + unsigned long last_scale_update;
> +};
> +
> +static DEFINE_PER_CPU_SHARED_ALIGNED(struct amu_cntr_sample, cpu_amu_samples);
> +
> void update_freq_counters_refs(void)
> {
> - this_cpu_write(arch_core_cycles_prev, read_corecnt());
> - this_cpu_write(arch_const_cycles_prev, read_constcnt());
> + struct amu_cntr_sample *amu_sample = this_cpu_ptr(&cpu_amu_samples);
> +
> + amu_sample->arch_core_cycles_prev = read_corecnt();
> + amu_sample->arch_const_cycles_prev = read_constcnt();
> }
>
> static inline bool freq_counters_valid(int cpu)
> {
> + struct amu_cntr_sample *amu_sample = per_cpu_ptr(&cpu_amu_samples, cpu);
> +
> if ((cpu >= nr_cpu_ids) || !cpumask_test_cpu(cpu, cpu_present_mask))
> return false;
>
> @@ -108,8 +119,8 @@ static inline bool freq_counters_valid(int cpu)
> return false;
> }
>
> - if (unlikely(!per_cpu(arch_const_cycles_prev, cpu) ||
> - !per_cpu(arch_core_cycles_prev, cpu))) {
> + if (unlikely(!amu_sample->arch_const_cycles_prev ||
> + !amu_sample->arch_core_cycles_prev)) {
> pr_debug("CPU%d: cycle counters are not enabled.\n", cpu);
> return false;
> }
> @@ -152,17 +163,22 @@ void freq_inv_set_max_ratio(int cpu, u64 max_rate)
>
> static void amu_scale_freq_tick(void)
> {
> + struct amu_cntr_sample *amu_sample = this_cpu_ptr(&cpu_amu_samples);
> u64 prev_core_cnt, prev_const_cnt;
> u64 core_cnt, const_cnt, scale;
>
> - prev_const_cnt = this_cpu_read(arch_const_cycles_prev);
> - prev_core_cnt = this_cpu_read(arch_core_cycles_prev);
> + prev_const_cnt = amu_sample->arch_const_cycles_prev;
> + prev_core_cnt = amu_sample->arch_core_cycles_prev;
>
> update_freq_counters_refs();
>
> - const_cnt = this_cpu_read(arch_const_cycles_prev);
> - core_cnt = this_cpu_read(arch_core_cycles_prev);
> + const_cnt = amu_sample->arch_const_cycles_prev;
> + core_cnt = amu_sample->arch_core_cycles_prev;
>
> + /*
> + * This should not happen unless the AMUs have been reset and the
> + * counter values have not been restored - unlikely
> + */
> if (unlikely(core_cnt <= prev_core_cnt ||
> const_cnt <= prev_const_cnt))
> return;
> @@ -182,6 +198,8 @@ static void amu_scale_freq_tick(void)
>
> scale = min_t(unsigned long, scale, SCHED_CAPACITY_SCALE);
> this_cpu_write(arch_freq_scale, (unsigned long)scale);
> +
> + amu_sample->last_scale_update = jiffies;
> }
>
> static struct scale_freq_data amu_sfd = {
> @@ -189,6 +207,77 @@ static struct scale_freq_data amu_sfd = {
> .set_freq_scale = amu_scale_freq_tick,
> };
>
> +static __always_inline bool amu_fie_cpu_supported(unsigned int cpu)
> +{
> + return cpumask_available(amu_fie_cpus) &&
> + cpumask_test_cpu(cpu, amu_fie_cpus);
> +}
> +
> +#define AMU_SAMPLE_EXP_MS 20
> +
> +int arch_freq_get_on_cpu(int cpu)
> +{
> + struct amu_cntr_sample *amu_sample;
> + unsigned int start_cpu = cpu;
> + unsigned long last_update;
> + unsigned int freq = 0;
> + u64 scale;
> +
> + if (!amu_fie_cpu_supported(cpu) || !arch_scale_freq_ref(cpu))
> + return -EOPNOTSUPP;
> +
> +retry:
> + amu_sample = per_cpu_ptr(&cpu_amu_samples, cpu);
> +
> + last_update = amu_sample->last_scale_update;
> +
> + /*
> + * For those CPUs that are in full dynticks mode, or those that have
> + * not seen tick for a while, try an alternative source for the counters
> + * (and thus freq scale), if available, for given policy: this boils
> + * down to identifying an active cpu within the same freq domain, if any.
> + */
> + if (!housekeeping_cpu(cpu, HK_TYPE_TICK) ||
> + time_is_before_jiffies(last_update + msecs_to_jiffies(AMU_SAMPLE_EXP_MS))) {
> + struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
> + int ref_cpu = cpu;
> +
> + if (!policy)
> + return -EINVAL;
> +
> + if (!cpumask_intersects(policy->related_cpus,
> + housekeeping_cpumask(HK_TYPE_TICK))) {
> + cpufreq_cpu_put(policy);
> + return -EOPNOTSUPP;
> + }
> +
> +
> + do {
> + ref_cpu = cpumask_next_wrap(ref_cpu, policy->cpus,
> + start_cpu, false);
> +
> + } while (ref_cpu < nr_cpu_ids && idle_cpu(ref_cpu));
> +
> + cpufreq_cpu_put(policy);
> +
> + if (ref_cpu >= nr_cpu_ids)
> + /* No alternative to pull info from */
> + return -EAGAIN;
> +
> + cpu = ref_cpu;
> + goto retry;
> + }
> + /*
> + * Reversed computation to the one used to determine
> + * the arch_freq_scale value
> + * (see amu_scale_freq_tick for details)
> + */
> + scale = arch_scale_freq_capacity(cpu);
> + freq = scale * arch_scale_freq_ref(cpu);
> + freq >>= SCHED_CAPACITY_SHIFT;
> + return freq;
> +}
> +
> static void amu_fie_setup(const struct cpumask *cpus)
> {
> int cpu;
> --
> 2.25.1
>