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@arm.com>
Signed-off-by: Beata Michalska <beata.michalska@arm.com>
---
 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 e475ec2705e1..2c002d2c3e0b 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),
+	 * 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

Hi Beata,

On 03/06/2024 16:21, 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@arm.com>
> Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> ---
>   arch/arm64/kernel/topology.c | 110 +++++++++++++++++++++++++++++++----
>   1 file changed, 100 insertions(+), 10 deletions(-)
...
> +
> +#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),
> +	 * 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))) {
One question here.

The 2nd condition, providing the addtional code in patch 4, would be:
(!idle_cpu(cpu) && time_is_before_jiffies(last_update + 
msecs_to_jiffies(AMU_SAMPLE_EXP_MS)))
That means trying another cpu in the same freq domain if this cpu is 
running and not having a tick recently.

In this case, if it fails to find an alternative cpu in the following 
code, can it just jump to the calculation
part and return an 'old' frequency rather than return 0?
The freq here won't be older than the freq when the cpu went idle last 
time -- yet the freq before last idle
is returned if the cpu were idle (patch 4).

> +		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);
start_cpu is only used here. looks like we can s/start_cpu/cpu/ and 
remove its definition above?
> +
> +		} 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;

On Wed, Aug 14, 2024 at 02:46:16PM +0800, Jie Zhan wrote:
> Hi Beata,
Hi Jie,
> 
> On 03/06/2024 16:21, 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@arm.com>
> > Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> > ---
> >   arch/arm64/kernel/topology.c | 110 +++++++++++++++++++++++++++++++----
> >   1 file changed, 100 insertions(+), 10 deletions(-)
> ...
> > +
> > +#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),
> > +	 * 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))) {
> One question here.
> 
> The 2nd condition, providing the addtional code in patch 4, would be:
> (!idle_cpu(cpu) && time_is_before_jiffies(last_update +
> msecs_to_jiffies(AMU_SAMPLE_EXP_MS)))
> That means trying another cpu in the same freq domain if this cpu is running
> and not having a tick recently.
> 
> In this case, if it fails to find an alternative cpu in the following code,
> can it just jump to the calculation
> part and return an 'old' frequency rather than return 0?
> The freq here won't be older than the freq when the cpu went idle last time
> -- yet the freq before last idle
> is returned if the cpu were idle (patch 4).
To be fair, I will be dropping the idle_cpu check from this condition so that
we do keep the cap on the validity of the scale factor: meaning if the cpu
remains idle for longer than assumed 20ms we will either look for alternative
or return 0. I'm not entirely convinced returning somewhat stale information on
the freq for CPUs that remain idle for a while will be useful.

One note here: as per discussion in [1] this functionality will be moved to new
sysfs attribute so it will no longer be used via scaling_cur_freq.


> 
> > +		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);
> start_cpu is only used here. looks like we can s/start_cpu/cpu/ and remove
> its definition above?
It is indeed but it is needed for the cpumask_next_wrap to know when to stop
the iteration after wrapping.

---
[1] https://lore.kernel.org/all/20240603081331.3829278-1-beata.michalska@arm.com/
---

BR
Beata
> > +
> > +		} 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;
>

On 26/08/2024 15:23, Beata Michalska wrote:
> On Wed, Aug 14, 2024 at 02:46:16PM +0800, Jie Zhan wrote:
>> Hi Beata,
> Hi Jie,
>> On 03/06/2024 16:21, 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@arm.com>
>>> Signed-off-by: Beata Michalska <beata.michalska@arm.com>
>>> ---
>>>    arch/arm64/kernel/topology.c | 110 +++++++++++++++++++++++++++++++----
>>>    1 file changed, 100 insertions(+), 10 deletions(-)
>> ...
>>> +
>>> +#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),
>>> +	 * 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))) {
>> One question here.
>>
>> The 2nd condition, providing the addtional code in patch 4, would be:
>> (!idle_cpu(cpu) && time_is_before_jiffies(last_update +
>> msecs_to_jiffies(AMU_SAMPLE_EXP_MS)))
>> That means trying another cpu in the same freq domain if this cpu is running
>> and not having a tick recently.
>>
>> In this case, if it fails to find an alternative cpu in the following code,
>> can it just jump to the calculation
>> part and return an 'old' frequency rather than return 0?
>> The freq here won't be older than the freq when the cpu went idle last time
>> -- yet the freq before last idle
>> is returned if the cpu were idle (patch 4).
> To be fair, I will be dropping the idle_cpu check from this condition so that
> we do keep the cap on the validity of the scale factor: meaning if the cpu
> remains idle for longer than assumed 20ms we will either look for alternative
> or return 0. I'm not entirely convinced returning somewhat stale information on
> the freq for CPUs that remain idle for a while will be useful.
>
> One note here: as per discussion in [1] this functionality will be moved to new
> sysfs attribute so it will no longer be used via scaling_cur_freq.
>
>
>>> +		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);
>> start_cpu is only used here. looks like we can s/start_cpu/cpu/ and remove
>> its definition above?
> It is indeed but it is needed for the cpumask_next_wrap to know when to stop
> the iteration after wrapping.
>
> ---
> [1] https://lore.kernel.org/all/20240603081331.3829278-1-beata.michalska@arm.com/
> ---
>
> BR
> Beata
>
Sure, thanks. Looking forward to the next version.

Jie

I apologize for the late review. This series dropped off my radar. I will test
this on an AmpereOne system.

On Mon, Jun 03, 2024 at 09:21:53AM +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@arm.com>
>Signed-off-by: Beata Michalska <beata.michalska@arm.com>
>---
> 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 e475ec2705e1..2c002d2c3e0b 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)

Could this be renamed to update_amu_cntr_sample() for more clarity?

> {
>-	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();

I think it would be better to update amp_sample->last_update here. update_freq_counters_refs
is the only way to update the amu_sample. So it should be safer to update the whole structure
in this method.

> }
>
> 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;

Please see the comment above. I think this line could be moved to
update_freq_counters_refs method.

> }
>
> 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),
>+	 * 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
>

On Wed, Jul 10, 2024 at 10:44:41AM -0700, Vanshidhar Konda wrote:
> I apologize for the late review. This series dropped off my radar. I will test
> this on an AmpereOne system.
> 
> On Mon, Jun 03, 2024 at 09:21:53AM +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@arm.com>
> > Signed-off-by: Beata Michalska <beata.michalska@arm.com>
> > ---
> > 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 e475ec2705e1..2c002d2c3e0b 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)
> 
> Could this be renamed to update_amu_cntr_sample() for more clarity?
I guess it could though to be fair I'd be more inclined to rename the struct
itself: this function updates the cached counter values that are being used
as a most recent ones but also as reference when calculating the delta
between the readings, so I'd say the naming here is pretty accurate.
> 
> > {
> > -	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();
> 
> I think it would be better to update amp_sample->last_update here. update_freq_counters_refs
> is the only way to update the amu_sample. So it should be safer to update the whole structure
> in this method.
Right, so the amu_sample->last_update is actually referring to updating the
scale factor, not the counter values per-se, even though one cannot be updated
without the other; so I'd rather keep those separated: the update time is being
used to determine whether the last known freq scale is still somewhat relevant.
If that would eliminate the confusion I can rename that filed.
> 
> > }
> > 
> > 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;
> 
> Please see the comment above. I think this line could be moved to
> update_freq_counters_refs method.
> 
> > }
> > 
> > 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),
> > +	 * 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
> >