Re: [PATCH v9 4/5] arm64: Provide an AMU-based version of arch_freq_get_on_cpu

[Beata Michalska <beata.michalska@arm.com>]

On Fri, Jan 24, 2025 at 10:13:30AM +0530, Prasanna Kumar T S M wrote:
> 
> On 21-01-2025 14:14, 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 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@arm.com>
> > Signed-off-by: Beata Michalska <beata.michalska@arm.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..5f5738b174c7 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_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, or 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: 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 -EINVAL;
> > +
> > +		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 -EAGAIN;
> > +
> > +		cpu = ref_cpu;
> > +		goto retry;
> 
> If you are going to spin a new revision, can you use while loop instead of
> using goto for looping? This will help improve the readability.
Can do, I guess, if you believe it will be more readable that way - me myself
slightly hesitating about that.

---
BR
Beata
> 
> > +	}
> > +	/*
> > +	 * 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;