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Date: Wed, 13 Mar 2024 22:47:36 +0100
From: Beata Michalska <beata.michalska@arm.com>
To: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	ionela.voinescu@arm.com, sudeep.holla@arm.com, will@kernel.org,
	catalin.marinas@arm.com, vincent.guittot@linaro.org,
	sumitg@nvidia.com, yang@os.amperecomputing.com,
	lihuisong@huawei.com
Subject: Re: [PATCH v3 2/3] arm64: Provide an AMU-based version of
 arch_freq_get_on_cpu
Message-ID: <ZfIe-JpW-DnFZYxJ@arm.com>
References: <20240312083431.3239989-1-beata.michalska@arm.com>
 <20240312083431.3239989-3-beata.michalska@arm.com>
 <q4pc63l43mtp56qim6s5riwu25cgvdqfchg7jbqhhcixkyqs7i@mlqqgd5hxnec>
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In-Reply-To: <q4pc63l43mtp56qim6s5riwu25cgvdqfchg7jbqhhcixkyqs7i@mlqqgd5hxnec>

On Tue, Mar 12, 2024 at 07:12:37PM -0700, Vanshidhar Konda wrote:
> On Tue, Mar 12, 2024 at 08:34:30AM +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);
> > 
> > 	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
> 
> /resroted/restored
> 
> > +	 */
> > 	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));
> > +
> > +	/*
> > +	 * 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));
> > +
> 
> Could you help me understand why getting the frequency from another
> housekeeping cpu would be a better than returning 0? Wouldn't different
> CPUs in the HK_TYPE_TICK domain be running at independent frequencies?
> May be adding this explanation to the patch commit message would help
> people who look at this in the future?

If the last AMU sample taken lost its assumed validity, we try another cpu within
the same frequency domain, choosing housekeeping cpu as one that might have seen
the tick within the last, assumed, 20ms. We stick to the cpus withn the same
policy, and thus same frequency domain which means those cpus do operate at the
same frequency. Now, in case of per-core dvfs this will bail out with '0' as the
policy->cpus will contain single CPU. Having said that, this code is bogus as it
does not handle needed wrapping in case currently considered cpu is the last one
in this policy cpus mask - will send an update soon.

I will also try to make the comment above  more readable.

---
BR
Beata
> 
> Thanks,
> Vanshi
> 
> > +		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
> >