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

[Prasanna Kumar T S M <ptsm@linux.microsoft.com>]

On 28-01-2025 13:46, Beata Michalska wrote:
> 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.

Feel free to pick whichever option you feel is best. Don't spin a new 
version just to change this.

I missed adding this in my previous email.

Looks good to me.
Reviewed-by: Prasanna Kumar T S M <ptsm@linux.microsoft.com>.

>
> ---
> 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;