Re: [PATCH v3 2/3] arm64: Provide an AMU-based version of arch_freq_get_on_cpu

[Ionela Voinescu <ionela.voinescu@arm.com>]

Hi Beata,

Thank you for the patches!

On Tuesday 12 Mar 2024 at 08:34:30 (+0000), 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
> on supported platforms 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@arm.com>
> Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> ---
>  arch/arm64/kernel/topology.c | 103 +++++++++++++++++++++++++++++++----
>  1 file changed, 92 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
> index 1a2c72f3e7f8..42cb19c31719 100644
> --- a/arch/arm64/kernel/topology.c
> +++ b/arch/arm64/kernel/topology.c
> @@ -17,6 +17,8 @@
>  #include <linux/cpufreq.h>
>  #include <linux/init.h>
>  #include <linux/percpu.h>
> +#include <linux/sched/isolation.h>
> +#include <linux/seqlock_types.h>
>  
>  #include <asm/cpu.h>
>  #include <asm/cputype.h>
> @@ -88,18 +90,31 @@ 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;
> +	seqcount_t	seq;
> +};
> +
> +static DEFINE_PER_CPU_SHARED_ALIGNED(struct amu_cntr_sample, cpu_amu_samples) = {
> +	.seq = SEQCNT_ZERO(cpu_amu_samples.seq)
> +};
> +
>  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 +123,8 @@ 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;
>  	}
> @@ -152,20 +167,27 @@ 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;
> +
> +	write_seqcount_begin(&amu_sample->seq);

The critical section here does not need to be this extensive, right?

The arch_freq_get_on_cpu() function only uses the frequency scale factor
and the last_update value, so this need only be placed above
"this_cpu_write(arch_freq_scale,..", if I'm not missing anything.

>  
>  	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 resroted - unlikely
> +	 */
>  	if (unlikely(core_cnt <= prev_core_cnt ||
>  		     const_cnt <= prev_const_cnt))
> -		return;
> +		goto leave;
>  
>  	/*
>  	 *	    /\core    arch_max_freq_scale
> @@ -182,6 +204,10 @@ 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;
> +leave:
> +	write_seqcount_end(&amu_sample->seq);
>  }
>  
>  static struct scale_freq_data amu_sfd = {
> @@ -189,6 +215,61 @@ static struct scale_freq_data amu_sfd = {
>  	.set_freq_scale = amu_scale_freq_tick,
>  };
>  
> +#define AMU_SAMPLE_EXP_MS	20
> +
> +unsigned int arch_freq_get_on_cpu(int cpu)
> +{
> +	struct amu_cntr_sample *amu_sample;
> +	unsigned long last_update;
> +	unsigned int seq;
> +	unsigned int freq;
> +	u64 scale;
> +
> +	if (!cpumask_test_cpu(cpu, amu_fie_cpus) || !arch_scale_freq_ref(cpu))
> +		return 0;
> +
> +retry:
> +	amu_sample = per_cpu_ptr(&cpu_amu_samples, cpu);
> +
> +	do {
> +		seq = raw_read_seqcount_begin(&amu_sample->seq);
> +		last_update = amu_sample->last_update;
> +	} while (read_seqcount_retry(&amu_sample->seq, seq));

Related to the point above, this retry loop should also contain
"scale = arch_scale_freq_capacity(cpu)", otherwise there's no much point
for synchronisation, as far as I can tell.

For x86, arch_freq_get_on_cpu() uses the counter deltas and it would be
bad if values from different ticks would be used. But here the only
benefit of synchronisation is to make sure that we're using the scale
factor computed at the last update time. For us, even skipping on the
synchronisation logic would still be acceptable, as we'd be ensuring that
there was a tick in the past 20ms and we'd always use the most recent
value of the frequency scale factor.

Hope it helps,
Ionela.

> +
> +	/*
> +	 * 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),
> +	 * if available for given policy
> +	 */
> +	if (time_is_before_jiffies(last_update + msecs_to_jiffies(AMU_SAMPLE_EXP_MS))) {
> +		struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
> +		int ref_cpu = nr_cpu_ids;
> +
> +		if (cpumask_intersects(housekeeping_cpumask(HK_TYPE_TICK),
> +				       policy->cpus))
> +			ref_cpu = cpumask_nth_and(cpu, policy->cpus,
> +						  housekeeping_cpumask(HK_TYPE_TICK));
> +
> +		cpufreq_cpu_put(policy);
> +		if (ref_cpu >= nr_cpu_ids || ref_cpu == cpu)
> +			/* No alternative to pull info from */
> +			return 0;
> +		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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