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Message-ID: <Zv8PKlr_PJgxazro@arm.com>
Date: Thu, 3 Oct 2024 23:39:54 +0200
From: Beata Michalska <beata.michalska@....com>
To: Vanshidhar Konda <vanshikonda@...amperecomputing.com>
Cc: Sumit Gupta <sumitg@...dia.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,
	yang@...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

On Thu, Sep 26, 2024 at 04:21:14PM -0700, Vanshidhar Konda wrote:
> On Thu, Sep 26, 2024 at 12:34:01PM GMT, Beata Michalska wrote:
> > On Tue, Sep 17, 2024 at 05:41:09PM +0530, Sumit Gupta wrote:
> > > Hi Beata,
> > Hi Sumit,
> > > 
> > > Thank you for the patches.
> > Thank you for having a look at those.
> > > 
> > > 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?
> > Yeah, might be.
> > > 
> > > > +        * 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;
> > >       }
> > True, we could but then this case is covered by cpumask_next_wrap
> > which for single-cpu policies will render the ref_cpu invalid,
> > so policy_is_shared check seemed unnecessary.
> > > 
> > > > +               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);
> > >   ----
> > > 
> > This boils down to the question what that function, and the information it
> > provides, represent really. The 'unknown' here simply says the CPU has been idle
> > for a while and as such the frequency data is a bit stale and it does not
> > represent the average freq the CPU is actually running at anymore, which is
> > the intention here really. Or, that the given CPU is a non-housekeeping one.
> > Either way I believe this is a useful information, instead of providing
> > stale data with no indication on whether the frequency is really the 'current'
> > one or not.
> > 
> > If that is somehow undesirable we can discuss this further, though I'd rather
> > avoid exposing an interface where the feedback provided is open to
> > interpretation at all times.
> 
> Would it make sense to identify that the frequency reporting is unknown due to
> cpu being idle vs some other issue like being a non-housekeeping CPU? Would
> returning a value of 0 make it easier for tools to represent that the CPU is
> currently idle?
That is an option.
Another one would be to return an error for those cases. This would make it
easier to distinguish between valid frequency &/| idle CPU vs tickless CPU
(EINVAL vs ENOENT) ?

---
BR
Beata
> 
> Thanks,
> Vanshidhar
> 
> > 
> > ---
> > Best Regards
> > Beata
> > > 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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