[PATCH v4 2/4] arm64: Provide an AMU-based version of arch_freq_get_on_cpu

[Beata Michalska]

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 at arm.com>
Signed-off-by: Beata Michalska <beata.michalska at arm.com>
---
 arch/arm64/kernel/topology.c | 112 +++++++++++++++++++++++++++++++----
 1 file changed, 102 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index 1a2c72f3e7f8..b03fe8617721 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 +207,80 @@ 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;
+	cpumask_var_t ref_cpumask = NULL;
+	unsigned long last_update;
+	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);
+
+	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_is_shared(policy)) {
+			cpufreq_cpu_put(policy);
+			return 0;
+		}
+
+		if (!ref_cpumask) {
+			if (!zalloc_cpumask_var(&ref_cpumask, GFP_KERNEL)) {
+				cpufreq_cpu_put(policy);
+				return 0;
+			}
+
+			cpumask_copy(ref_cpumask, policy->cpus);
+		}
+
+		cpufreq_cpu_put(policy);
+
+		do {
+			cpumask_clear_cpu(ref_cpu, ref_cpumask);
+			ref_cpu = cpumask_first(ref_cpumask);
+
+		} while (ref_cpu < nr_cpu_ids && idle_cpu(ref_cpu));
+
+		if (ref_cpu >= nr_cpu_ids) {
+			/* No alternative to pull info from */
+			free_cpumask_var(ref_cpumask);
+			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;
+
+	free_cpumask_var(ref_cpumask);
+
+	return freq;
+}
+
 static void amu_fie_setup(const struct cpumask *cpus)
 {
 	int cpu;
-- 
2.25.1