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

Prasanna Kumar T S M
2025-01-28T08:52:46Z

On 28-01-2025 13:46, Beata Michalska wrote:
> 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.

Feel free to pick whichever option you feel is best. Don't spin a new
version just to change this.

I missed adding this in my previous email.

Looks good to me.
Reviewed-by: Prasanna Kumar T S M <ptsm@linux.microsoft.com>.

>
> ---
> 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;