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Message-ID: <ylcfqw4swz6xjxxfoyljyifs4ibbueywogqxusxfz3a3fgh3du@cfaajchbwgvn>
Date: Thu, 26 Sep 2024 16:21:14 -0700
From: Vanshidhar Konda <vanshikonda@...amperecomputing.com>
To: Beata Michalska <beata.michalska@....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 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?
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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