Re: [PATCH v5 6/7] arm64: use activity monitors for frequency invariance

Lukasz Luba
2020-02-27T08:44:02Z

On 2/26/20 1:29 PM, Ionela Voinescu wrote:
> The Frequency Invariance Engine (FIE) is providing a frequency
> scaling correction factor that helps achieve more accurate
> load-tracking.
>
> So far, for arm and arm64 platforms, this scale factor has been
> obtained based on the ratio between the current frequency and the
> maximum supported frequency recorded by the cpufreq policy. The
> setting of this scale factor is triggered from cpufreq drivers by
> calling arch_set_freq_scale. The current frequency used in computation
> is the frequency requested by a governor, but it may not be the
> frequency that was implemented by the platform.
>
> This correction factor can also be obtained using a core counter and a
> constant counter to get information on the performance (frequency based
> only) obtained in a period of time. This will more accurately reflect
> the actual current frequency of the CPU, compared with the alternative
> implementation that reflects the request of a performance level from
> the OS.
>
> Therefore, implement arch_scale_freq_tick to use activity monitors, if
> present, for the computation of the frequency scale factor.
>
> The use of AMU counters depends on:
> - CONFIG_ARM64_AMU_EXTN - depents on the AMU extension being present
> - CONFIG_CPU_FREQ - the current frequency obtained using counter
> information is divided by the maximum frequency obtained from the
> cpufreq policy.
>
> While it is possible to have a combination of CPUs in the system with
> and without support for activity monitors, the use of counters for
> frequency invariance is only enabled for a CPU if all related CPUs
> (CPUs in the same frequency domain) support and have enabled the core
> and constant activity monitor counters. In this way, there is a clear
> separation between the policies for which arch_set_freq_scale (cpufreq
> based FIE) is used, and the policies for which arch_scale_freq_tick
> (counter based FIE) is used to set the frequency scale factor. For
> this purpose, a late_initcall_sync is registered to trigger validation
> work for policies that will enable or disable the use of AMU counters
> for frequency invariance. If CONFIG_CPU_FREQ is not defined, the use
> of counters is enabled on all CPUs only if all possible CPUs correctly
> support the necessary counters.
>
> Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Sudeep Holla <sudeep.holla@arm.com>
> ---
> arch/arm64/include/asm/topology.h | 9 ++
> arch/arm64/kernel/cpufeature.c | 4 +
> arch/arm64/kernel/topology.c | 180 ++++++++++++++++++++++++++++++
> drivers/base/arch_topology.c | 12 ++
> include/linux/arch_topology.h | 2 +
> 5 files changed, 207 insertions(+)
>
> diff --git a/arch/arm64/include/asm/topology.h b/arch/arm64/include/asm/topology.h
> index a4d945db95a2..21d4d40d6243 100644
> --- a/arch/arm64/include/asm/topology.h
> +++ b/arch/arm64/include/asm/topology.h
> @@ -16,6 +16,15 @@ int pcibus_to_node(struct pci_bus *bus);
>
> #include <linux/arch_topology.h>
>
> +#ifdef CONFIG_ARM64_AMU_EXTN
> +/*
> + * Replace task scheduler's default counter-based
> + * frequency-invariance scale factor setting.
> + */
> +void topology_scale_freq_tick(void);
> +#define arch_scale_freq_tick topology_scale_freq_tick
> +#endif /* CONFIG_ARM64_AMU_EXTN */
> +
> /* Replace task scheduler's default frequency-invariant accounting */
> #define arch_scale_freq_capacity topology_get_freq_scale
>
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 60cebc071603..b8ec6c544d32 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1241,12 +1241,16 @@ bool cpu_has_amu_feat(int cpu)
> return cpumask_test_cpu(cpu, &amu_cpus);
> }
>
> +/* Initialize the use of AMU counters for frequency invariance */
> +extern void init_cpu_freq_invariance_counters(void);
> +
> static void cpu_amu_enable(struct arm64_cpu_capabilities const *cap)
> {
> if (has_cpuid_feature(cap, SCOPE_LOCAL_CPU)) {
> pr_info("detected CPU%d: Activity Monitors Unit (AMU)\n",
> smp_processor_id());
> cpumask_set_cpu(smp_processor_id(), &amu_cpus);
> + init_cpu_freq_invariance_counters();
> }
> }
>
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index fa9528dfd0ce..0801a0f3c156 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -14,6 +14,7 @@
> #include <linux/acpi.h>
> #include <linux/arch_topology.h>
> #include <linux/cacheinfo.h>
> +#include <linux/cpufreq.h>
> #include <linux/init.h>
> #include <linux/percpu.h>
>
> @@ -120,4 +121,183 @@ int __init parse_acpi_topology(void)
> }
> #endif
>
> +#ifdef CONFIG_ARM64_AMU_EXTN
>
> +#undef pr_fmt
> +#define pr_fmt(fmt) "AMU: " fmt
> +
> +static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, arch_max_freq_scale);
> +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;
> +
> +/* Initialize counter reference per-cpu variables for the current CPU */
> +void init_cpu_freq_invariance_counters(void)
> +{
> + this_cpu_write(arch_core_cycles_prev,
> + read_sysreg_s(SYS_AMEVCNTR0_CORE_EL0));
> + this_cpu_write(arch_const_cycles_prev,
> + read_sysreg_s(SYS_AMEVCNTR0_CONST_EL0));
> +}
> +
> +static int validate_cpu_freq_invariance_counters(int cpu)
> +{
> + u64 max_freq_hz, ratio;
> +
> + if (!cpu_has_amu_feat(cpu)) {
> + pr_debug("CPU%d: counters are not supported.\n", cpu);
> + return -EINVAL;
> + }
> +
> + if (unlikely(!per_cpu(arch_const_cycles_prev, cpu) ||
> + !per_cpu(arch_core_cycles_prev, cpu))) {
> + pr_debug("CPU%d: cycle counters are not enabled.\n", cpu);
> + return -EINVAL;
> + }
> +
> + /* Convert maximum frequency from KHz to Hz and validate */
> + max_freq_hz = cpufreq_get_hw_max_freq(cpu) * 1000;
> + if (unlikely(!max_freq_hz)) {
> + pr_debug("CPU%d: invalid maximum frequency.\n", cpu);
> + return -EINVAL;
> + }
> +
> + /*
> + * Pre-compute the fixed ratio between the frequency of the constant
> + * counter and the maximum frequency of the CPU.
> + *
> + * const_freq
> + * arch_max_freq_scale = ---------------- * SCHED_CAPACITY_SCALE²
> + * cpuinfo_max_freq
> + *
> + * We use a factor of 2 * SCHED_CAPACITY_SHIFT -> SCHED_CAPACITY_SCALE²
> + * in order to ensure a good resolution for arch_max_freq_scale for
> + * very low arch timer frequencies (down to the KHz range which should
> + * be unlikely).
> + */
> + ratio = (u64)arch_timer_get_rate() << (2 * SCHED_CAPACITY_SHIFT);
> + ratio = div64_u64(ratio, max_freq_hz);
> + if (!ratio) {
> + WARN_ONCE(1, "System timer frequency too low.\n");
> + return -EINVAL;
> + }
> +
> + per_cpu(arch_max_freq_scale, cpu) = (unsigned long)ratio;
> +
> + return 0;
> +}
> +
> +static inline bool
> +enable_policy_freq_counters(int cpu, cpumask_var_t valid_cpus)
> +{
> + struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
> +
> + if (!policy) {
> + pr_debug("CPU%d: No cpufreq policy found.\n", cpu);
> + return false;
> + }
> +
> + if (cpumask_subset(policy->related_cpus, valid_cpus))
> + cpumask_or(amu_fie_cpus, policy->related_cpus,
> + amu_fie_cpus);
> +
> + cpufreq_cpu_put(policy);
> +
> + return true;
> +}
> +
> +static DEFINE_STATIC_KEY_FALSE(amu_fie_key);
> +#define amu_freq_invariant() static_branch_unlikely(&amu_fie_key)
> +
> +static int __init init_amu_fie(void)
> +{
> + cpumask_var_t valid_cpus;
> + bool have_policy = false;
> + int ret = 0;
> + int cpu;
> +
> + if (!zalloc_cpumask_var(&valid_cpus, GFP_KERNEL))
> + return -ENOMEM;
> +
> + if (!zalloc_cpumask_var(&amu_fie_cpus, GFP_KERNEL)) {
> + ret = -ENOMEM;
> + goto free_valid_mask;
> + }
> +
> + for_each_present_cpu(cpu) {
> + if (validate_cpu_freq_invariance_counters(cpu))
> + continue;
> + cpumask_set_cpu(cpu, valid_cpus);
> + have_policy |= enable_policy_freq_counters(cpu, valid_cpus);
> + }
> +
> + /*
> + * If we are not restricted by cpufreq policies, we only enable
> + * the use of the AMU feature for FIE if all CPUs support AMU.
> + * Otherwise, enable_policy_freq_counters has already enabled
> + * policy cpus.
> + */
> + if (!have_policy && cpumask_equal(valid_cpus, cpu_present_mask))
> + cpumask_or(amu_fie_cpus, amu_fie_cpus, valid_cpus);
> +
> + if (!cpumask_empty(amu_fie_cpus)) {
> + pr_info("CPUs[%*pbl]: counters will be used for FIE.",
> + cpumask_pr_args(amu_fie_cpus));
> + static_branch_enable(&amu_fie_key);
> + }
> +
> +free_valid_mask:
> + free_cpumask_var(valid_cpus);
> +
> + return ret;
> +}
> +late_initcall_sync(init_amu_fie);
> +
> +bool arch_freq_counters_available(struct cpumask *cpus)
> +{
> + return amu_freq_invariant() &&
> + cpumask_subset(cpus, amu_fie_cpus);
> +}
> +
> +void topology_scale_freq_tick(void)
> +{
> + u64 prev_core_cnt, prev_const_cnt;
> + u64 core_cnt, const_cnt, scale;
> + int cpu = smp_processor_id();
> +
> + if (!amu_freq_invariant())
> + return;
> +
> + if (!cpumask_test_cpu(cpu, amu_fie_cpus))
> + return;
> +
> + const_cnt = read_sysreg_s(SYS_AMEVCNTR0_CONST_EL0);
> + core_cnt = read_sysreg_s(SYS_AMEVCNTR0_CORE_EL0);
> + prev_const_cnt = this_cpu_read(arch_const_cycles_prev);
> + prev_core_cnt = this_cpu_read(arch_core_cycles_prev);
> +
> + if (unlikely(core_cnt <= prev_core_cnt ||
> + const_cnt <= prev_const_cnt))
> + goto store_and_exit;
> +
> + /*
> + * /\core arch_max_freq_scale
> + * scale = ------- * --------------------
> + * /\const SCHED_CAPACITY_SCALE
> + *
> + * See validate_cpu_freq_invariance_counters() for details on
> + * arch_max_freq_scale and the use of SCHED_CAPACITY_SHIFT.
> + */
> + scale = core_cnt - prev_core_cnt;
> + scale *= this_cpu_read(arch_max_freq_scale);
> + scale = div64_u64(scale >> SCHED_CAPACITY_SHIFT,
> + const_cnt - prev_const_cnt);
> +
> + scale = min_t(unsigned long, scale, SCHED_CAPACITY_SCALE);
> + this_cpu_write(freq_scale, (unsigned long)scale);
> +
> +store_and_exit:
> + this_cpu_write(arch_core_cycles_prev, core_cnt);
> + this_cpu_write(arch_const_cycles_prev, const_cnt);
> +}
> +#endif /* CONFIG_ARM64_AMU_EXTN */
> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
> index 6119e11a9f95..8d63673c1689 100644
> --- a/drivers/base/arch_topology.c
> +++ b/drivers/base/arch_topology.c
> @@ -21,6 +21,10 @@
> #include <linux/sched.h>
> #include <linux/smp.h>
>
> +__weak bool arch_freq_counters_available(struct cpumask *cpus)
> +{
> + return false;
> +}
> DEFINE_PER_CPU(unsigned long, freq_scale) = SCHED_CAPACITY_SCALE;
>
> void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq,
> @@ -29,6 +33,14 @@ void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq,
> unsigned long scale;
> int i;
>
> + /*
> + * If the use of counters for FIE is enabled, just return as we don't
> + * want to update the scale factor with information from CPUFREQ.
> + * Instead the scale factor will be updated from arch_scale_freq_tick.
> + */
> + if (arch_freq_counters_available(cpus))
> + return;
> +
> scale = (cur_freq << SCHED_CAPACITY_SHIFT) / max_freq;
>
> for_each_cpu(i, cpus)
> diff --git a/include/linux/arch_topology.h b/include/linux/arch_topology.h
> index 3015ecbb90b1..1ccdddb541a7 100644
> --- a/include/linux/arch_topology.h
> +++ b/include/linux/arch_topology.h
> @@ -33,6 +33,8 @@ unsigned long topology_get_freq_scale(int cpu)
> return per_cpu(freq_scale, cpu);
> }
>
> +bool arch_freq_counters_available(struct cpumask *cpus);
> +
> struct cpu_topology {
> int thread_id;
> int core_id;
>

Looks good

Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>

Regards,
Lukasz