Re: [PATCH v5 3/5] arm64: Provide an AMU-based version of arch_freq_get_on_cpu

[Vanshidhar Konda]

On Wed, Apr 17, 2024 at 10:38:46AM +0100, 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 current 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@xxxxxxx>
> Signed-off-by: Beata Michalska <beata.michalska@xxxxxxx>
> ---
> arch/arm64/kernel/topology.c | 110 +++++++++++++++++++++++++++++++----
> 1 file changed, 100 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index 3c814a278534..475fdbf3032a 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_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_update = jiffies;
> }
>
> static struct scale_freq_data amu_sfd = {
> @@ -189,6 +207,78 @@ 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
> +
> +unsigned 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 0;
> +
> +retry:
> +	amu_sample = per_cpu_ptr(&cpu_amu_samples, cpu);
> +
> +	last_update = amu_sample->last_update;
> +
> +	/*
> +	 * For those CPUs that are in full dynticks mode,
> +	 * and those that have not seen tick for a while
> +	 * try an alternative source for the counters (and thus freq scale),

While testing this on AmpereOne system I found that the scaling_cur_freq
and cpufreq_cur_freq are inconsistent for nohz_full CPUs that are being
throttled (OS requested freq != HW provided freq).

For the test I ran stress-ng workload on 9 cores. All the other cores
are idle. I then forced the hardware to throttle the active cores - core
won't run at maximum frequency despite a request from the OS. Each core
has an independent cpufreq policy.

For the nohz_full CPUs since arch_freq_get_on_cpu bails out. In
show_scaling_cur_freq() the next check is to see if
cpufreq_driver->set_policy method is implemented. cppc_cpufreq does not
implement this method and we just end up returning the policy->cur
value. As discussed in other threads, it looks like we want the behavior
to be identical to x86 systems. In that case it seems like returning 0
from arch_freq_get_on_cpu is not going to be valid behavior.

Core    scaling_cur_freq cpufreq_cur_freq
[0]:    2700000          2700000
[1]:    2750000          2750000

nohz_full=2-7
[2]:    3200000          2691000
[3]:    3200000          2645000
[4]:    3200000          2731000
[5]:    3200000          2714000
[6]:    3200000          2466000
[7]:    3200000          2708000

isolcpus=8-11 (no workload applied to core 10-11)
[8]:    2700000          2700000
[9]:    2550000          2550000
[10]:   1046875          1046875
[11]:   1096875          1096875

Thanks,
Vanshi

> +	 * 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)
> +			goto leave;
> +
> +		if (!policy_is_shared(policy)) {
> +			cpufreq_cpu_put(policy);
> +			goto leave;
> +		}
> +
> +		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 */
> +			goto leave;
> +
> +		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;
> +leave:
> +	return freq;
> +}
> +
> static void amu_fie_setup(const struct cpumask *cpus)
> {
> 	int cpu;
> --
> 2.25.1