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Message-ID: <aa254516-968e-4665-bb5b-981c296ffc35@nvidia.com>
Date: Tue, 17 Sep 2024 17:41:09 +0530
From: Sumit Gupta <sumitg@...dia.com>
To: Beata Michalska <beata.michalska@....com>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-pm@...r.kernel.org>,
	<ionela.voinescu@....com>, <sudeep.holla@....com>, <will@...nel.org>,
	<catalin.marinas@....com>, <rafael@...nel.org>, <viresh.kumar@...aro.org>
CC: <yang@...amperecomputing.com>, <vanshikonda@...amperecomputing.com>,
	<lihuisong@...wei.com>, <zhanjie9@...ilicon.com>, linux-tegra
	<linux-tegra@...r.kernel.org>, Bibek Basu <bbasu@...dia.com>
Subject: Re: [PATCH v7 3/4] arm64: Provide an AMU-based version of
 arch_freq_avg_get_on_cpu

Hi Beata,

Thank you for the patches.

On 13/09/24 18:59, Beata Michalska wrote:
> External email: Use caution opening links or attachments
> 
> 
> 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@....com>
> Signed-off-by: Beata Michalska <beata.michalska@....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..22e510733336 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_avg_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, and those that have
'or those' to match with if condition?

> +        * 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 0;
> +

We can skip the rest of code if policy has a single cpu. AFAIR, one of 
the previous versions had similar check.

       if (!policy_is_shared(policy)) {
               cpufreq_cpu_put(policy);
               goto freq_comput;
       }

> +               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 0;
> +

The 'cpuinfo_avg_freq' node gives 'unknown' value for single CPU per 
policy as 'ref_cpu' increments to 'nr_cpu_ids'. We can use the same CPU 
instead of returning zero if no alternative CPU.

   # cat /sys/devices/system/cpu/cpu2/cpufreq/cpuinfo_avg_freq
   <unknown>

   ----
       if (ref_cpu >= nr_cpu_ids)
           /* Use same CPU if no alternative to pull info from */
           goto freq_comput;

     ..
   freq_comput:
     scale = arch_scale_freq_capacity(cpu);
     freq = scale * arch_scale_freq_ref(cpu);
   ----

Thank you,
Sumit Gupta

P.S. Will be on afk for next 2 weeks with no access to email. Please 
expect a delay in response.

> +               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
> 

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