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

Beata Michalska
2025-01-28T08:17:36Z

On Fri, Jan 24, 2025 at 10:13:30AM +0530, Prasanna Kumar T S M wrote:
>
> On 21-01-2025 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>
> > ---
> > 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;
>
> If you are going to spin a new revision, can you use while loop instead of
> using goto for looping? This will help improve the readability.
Can do, I guess, if you believe it will be more readable that way - me myself
slightly hesitating about that.

---
BR
Beata
>
> > + }
> > + /*
> > + * 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;