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Date: Tue, 12 Mar 2024 19:12:37 -0700
From: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
To: Beata Michalska <beata.michalska@arm.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: <q4pc63l43mtp56qim6s5riwu25cgvdqfchg7jbqhhcixkyqs7i@mlqqgd5hxnec>
References: <20240312083431.3239989-1-beata.michalska@arm.com>
 <20240312083431.3239989-3-beata.michalska@arm.com>
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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?

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
>