Hi Valentin,

Sorry for the delay in my reply and thank you very much for the review!

I will push v4 very soon with these changes.

On Monday 17 Feb 2020 at 16:59:24 (+0000), Valentin Schneider wrote:

quoted

+	 * Pre-compute the fixed ratio between the frequency of the constant
+	 * counter and the maximum frequency of the CPU.
+	 *
+	 *			      const_freq
+	 * arch_max_freq_scale =   ---------------- * SCHED_CAPACITY_SCALE²
+	 *			   cpuinfo_max_freq
+	 *
+	 * We use a factor of 2 * SCHED_CAPACITY_SHIFT -> SCHED_CAPACITY_SCALE²
+	 * in order to ensure a good resolution for arch_max_freq_scale for
+	 * very low arch timer frequencies (up to the KHz range which should be

                                            ^^^^^
<pedantic hat on>: s/up to/down to/

Done!

quoted

+	 * unlikely).
+	 */
+	ratio = (u64)arch_timer_get_rate() << (2 * SCHED_CAPACITY_SHIFT);
+	ratio = div64_u64(ratio, max_freq_hz);
+	if (!ratio) {
+		pr_err("System timer frequency too low.\n");

Should that be a WARN_ONCE() instead? If the arch timer freq is too low,
we'll end up spamming this message, since we go through this for all CPUs.

Done!

quoted

+		return -EINVAL;
+	}
+
+	per_cpu(arch_max_freq_scale, cpu) = (unsigned long)ratio;
+

It occurred to me that this isn't strictly speaking a per-CPU information as
it only depends on the max possible frequency. Not really worth bothering
about though, I think.

Yes, it depends on the max possible frequency of all CPUs in a frequency
domain. But I wanted to put this factor in a per-cpu variable in order
to be able to retrieve it faster in topology_scale_freq_tick, rather
than having to consider policies and related CPUs in that function.

quoted

+	return 0;
+}
+
+static inline int
+enable_policy_freq_counters(int cpu, cpumask_var_t valid_cpus)
+{
+	struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
+
+	if (!policy) {
+		pr_debug("CPU%d: No cpufreq policy found.\n", cpu);
+		return false;
+	}
+
+	if (cpumask_subset(policy->related_cpus, valid_cpus)) {
+		cpumask_or(amu_fie_cpus, policy->related_cpus,
+			   amu_fie_cpus);
+		pr_info("CPUs[%*pbl]: counters will be used for FIE.",
+			cpumask_pr_args(amu_fie_cpus));

Could we have a single print of all CPUs in one go? AIUI this will generate a
line per cpufreq policy. Maybe just something at the tail of init_amu_fie():

if (!cpumask_empty(amu_fie_cpus))
	pr_info(<blah>);

Done. I've used this location as well to set the static key that you've
suggested below.

quoted

+	}
+
+	cpufreq_cpu_put(policy);
+
+	return true;
+}
+
+static int __init init_amu_fie(void)
+{
+	cpumask_var_t valid_cpus;
+	bool have_policy = false;
+	int cpu;
+
+	if (!zalloc_cpumask_var(&valid_cpus, GFP_KERNEL) ||
+	    !zalloc_cpumask_var(&amu_fie_cpus, GFP_KERNEL))
+		return -ENOMEM;
+
+	for_each_possible_cpu(cpu) {
+		if (validate_cpu_freq_invariance_counters(cpu))
+			continue;
+		cpumask_set_cpu(cpu, valid_cpus);
+		have_policy = enable_policy_freq_counters(cpu, valid_cpus) ||
+			      have_policy;

What about:
		have_policy |= enable_policy_freq_counters(cpu, valid_cpus);

Done as well.

quoted

+	}
+
+	if (!have_policy) {
+		/*
+		 * If we are not restricted by cpufreq policies, we only enable
+		 * the use of the AMU feature for FIE if all CPUs support AMU.
+		 * Otherwise, enable_policy_freq_counters has already enabled
+		 * policy cpus.
+		 */
+		if (cpumask_equal(valid_cpus, cpu_possible_mask)) {

Mmm so I'm thinking what we want here is the cpu_present_mask rather than
the possible one. This is very corner-casy, but I think that if we fail to
boot a secondary, we'll have it possible but not present.

Yes, this is correct. It does depend on the stage it fails at: for
example if some feature checks fail, a CPU will not be marked in
cpu_present_mask (see cpu_die_early()), while the following will result
in possible == present.

---
[    0.056524] EFI services will not be available.
[    0.065690] smp: Bringing up secondary CPUs ...
[    0.098010] psci: failed to boot CPU1 (-22)
[    0.098037] CPU1: failed to boot: -22
[    0.130290] psci: failed to boot CPU2 (-22)
[    0.130315] CPU2: failed to boot: -22
[    0.162568] psci: failed to boot CPU3 (-22)
[    0.162594] CPU3: failed to boot: -22
[    0.194890] Detected PIPT I-cache on CPU4
[    0.194990] GICv3: CPU4: found redistributor 100 region
0:0x000000002f120000
[    0.195046] GICv3: CPU4: using allocated LPI pending table
@0x00000000fc0d0000
[    0.195133] CPU4: Booted secondary processor 0x0000000100
[0x410fd0f0]
[    0.227190] psci: failed to boot CPU5 (-22)
[    0.227412] CPU5: failed to boot: -22
[    0.259431] psci: failed to boot CPU6 (-22)
[    0.259522] CPU6: failed to boot: -22
[    0.291683] psci: failed to boot CPU7 (-22)
[    0.291709] CPU7: failed to boot: -22
[    0.291990] smp: Brought up 1 node, 2 CPUs  
[..]
root@buildroot:~# cat present
0-7
root@buildroot:~# cat possible
0-7

This failure happens while the CPU is being brought up (__cpu_up).
I'm not sure if this should result in set_cpu_present(cpu, 0) as well.
But it's unrelated to this..

In any case, your suggestion is valid and cpu_present_mask is better to
be used here.

While at it you could make the loop only target present CPUs, but I think the
one bit that matters is this check right here (!present should fail at
validate_cpu_freq_invariance_counters()).

Will change the loop as well. Thanks!

quoted

+			cpumask_or(amu_fie_cpus, amu_fie_cpus, valid_cpus);
+			pr_info("CPUs[%*pbl]: counters will be used for FIE.",
+				cpumask_pr_args(amu_fie_cpus));
+		}
+	}
+
+	free_cpumask_var(valid_cpus);
+
+	return 0;
+}
+late_initcall_sync(init_amu_fie);
+
+bool topology_cpu_freq_counters(struct cpumask *cpus)
+{
+	return cpumask_available(amu_fie_cpus) &&
+	       cpumask_subset(cpus, amu_fie_cpus);
+}
+
+void topology_scale_freq_tick(void)
+{
+	u64 prev_core_cnt, prev_const_cnt;
+	u64 core_cnt, const_cnt, scale;
+	int cpu = smp_processor_id();
+
+	if (!cpumask_available(amu_fie_cpus) ||
+	    !cpumask_test_cpu(cpu, amu_fie_cpus))
+		return;

It might be a good idea to have a static key to gate our entry into this
function - that way we can lessen our impact on older platforms (without AMUs)
running a recent kernel with CONFIG_ARM64_AMU_EXTN=y.

x86 does just that, if you look at their arch_scale_freq_tick()
implementation. FWIW I don't think we should bother with playing with the
key counter to count AMU-enabled CPUs, just enable it at startup if we have

quoted

1 such CPU and let the cpumask drive the rest.

In your check here, the static key check could replace the cpumask_available()
check. The static key could also be used for topology_cpu_freq_counters().

Very good idea! Done as well. Yes, the counter (number of AMU enabled
CPUs) would not be of much help for the moment.

quoted

+
+	const_cnt = read_sysreg_s(SYS_AMEVCNTR0_CONST_EL0);
+	core_cnt = read_sysreg_s(SYS_AMEVCNTR0_CORE_EL0);
+	prev_const_cnt = this_cpu_read(arch_const_cycles_prev);
+	prev_core_cnt = this_cpu_read(arch_core_cycles_prev);
+
+	if (unlikely(core_cnt <= prev_core_cnt ||
+		     const_cnt <= prev_const_cnt))
+		goto store_and_exit;
+
+	/*
+	 *	    /\core    arch_max_freq_scale
+	 * scale =  ------- * --------------------
+	 *	    /\const   SCHED_CAPACITY_SCALE
+	 *
+	 * We shift by SCHED_CAPACITY_SHIFT (divide by SCHED_CAPACITY_SCALE)
+	 * in order to compensate for the SCHED_CAPACITY_SCALE² factor in
+	 * arch_max_freq_scale (used to ensure its resolution) while keeping
+	 * the scale value in the 0-SCHED_CAPACITY_SCALE capacity range.
+	 */

A simple "See validate_cpu_freq_invariance_counters() for details on the
scale factor" would suffice wrt the shifting details.

Done!

Thank you,
Ionela.

quoted

+	scale = core_cnt - prev_core_cnt;
+	scale *= this_cpu_read(arch_max_freq_scale);
+	scale = div64_u64(scale >> SCHED_CAPACITY_SHIFT,
+			  const_cnt - prev_const_cnt);
+
+	scale = min_t(unsigned long, scale, SCHED_CAPACITY_SCALE);
+	this_cpu_write(freq_scale, (unsigned long)scale);
+
+store_and_exit:
+	this_cpu_write(arch_core_cycles_prev, core_cnt);
+	this_cpu_write(arch_const_cycles_prev, const_cnt);
+}
+#endif /* CONFIG_ARM64_AMU_EXTN */

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 1eb81f113786..1ab2b7503d63 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c

@@ -29,6 +29,14 @@ void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq,
 	unsigned long scale;
 	int i;
 
+	/*
+	 * If the use of counters for FIE is enabled, just return as we don't
+	 * want to update the scale factor with information from CPUFREQ.
+	 * Instead the scale factor will be updated from arch_scale_freq_tick.
+	 */
+	if (arch_cpu_freq_counters(cpus))
+		return;
+
 	scale = (cur_freq << SCHED_CAPACITY_SHIFT) / max_freq;
 
 	for_each_cpu(i, cpus)

diff --git a/include/linux/topology.h b/include/linux/topology.h
index eb2fe6edd73c..397aad6ae163 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h

@@ -227,5 +227,12 @@ static inline const struct cpumask *cpu_cpu_mask(int cpu)
 	return cpumask_of_node(cpu_to_node(cpu));
 }
 
+#ifndef arch_cpu_freq_counters
+static __always_inline
+bool arch_cpu_freq_counters(struct cpumask *cpus)
+{
+	return false;
+}
+#endif
 
 #endif /* _LINUX_TOPOLOGY_H */