Re: [PATCH v1 2/3] cpufreq: CPPC: Add per_cpu efficiency_class
From: Pierre Gondois
Date: Thu Mar 17 2022 - 12:06:56 EST
On 3/17/22 16:13, Marc Zyngier wrote:
On 2022-03-17 13:34, Pierre Gondois wrote:
In ACPI, describing power efficiency of CPUs can be done through the
following arm specific field:
ACPI 6.4, s5.2.12.14 'GIC CPU Interface (GICC) Structure',
'Processor Power Efficiency Class field':
Describes the relative power efficiency of the associated pro-
cessor. Lower efficiency class numbers are more efficient than
higher ones (e.g. efficiency class 0 should be treated as more
efficient than efficiency class 1). However, absolute values
of this number have no meaning: 2 isn’t necessarily half as
efficient as 1.
The efficiency_class field is stored in the GicC structure of the
ACPI MADT table and it's currently supported in Linux for arm64 only.
Thus, this new functionality is introduced for arm64 only.
To allow the cppc_cpufreq driver to know and preprocess the
efficiency_class values of all the CPUs, add a per_cpu efficiency_class
variable to store them. Also add a static efficiency_class_populated
to let the driver know efficiency_class values are usable and register
an artificial Energy Model (EM) based on normalized class values.
At least 2 different efficiency classes must be present,
otherwise there is no use in creating an Energy Model.
The efficiency_class values are squeezed in [0:#efficiency_class-1]
while conserving the order. For instance, efficiency classes of:
[111, 212, 250]
will be mapped to:
[0 (was 111), 1 (was 212), 2 (was 250)].
Each policy being independently registered in the driver, populating
the per_cpu efficiency_class is done only once at the driver
initialization. This prevents from having each policy re-searching the
efficiency_class values of other CPUs.
The patch also exports acpi_cpu_get_madt_gicc() to fetch the GicC
structure of the ACPI MADT table for each CPU.
Signed-off-by: Pierre Gondois <Pierre.Gondois@xxxxxxx>
---
arch/arm64/kernel/smp.c | 1 +
drivers/cpufreq/cppc_cpufreq.c | 55 ++++++++++++++++++++++++++++++++++
2 files changed, 56 insertions(+)
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 27df5c1e6baa..56637cbea5d6 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -512,6 +512,7 @@ struct acpi_madt_generic_interrupt
*acpi_cpu_get_madt_gicc(int cpu)
{
return &cpu_madt_gicc[cpu];
}
+EXPORT_SYMBOL(acpi_cpu_get_madt_gicc);
Why not EXPORT_SYMBOL_GPL()?
From what I understand, this could be made EXPORT_SYMBOL_GPL().
The only reason was that the other symbol exportation in the
file wasn't restricted to GPL.
/*
* acpi_map_gic_cpu_interface - parse processor MADT entry
diff --git a/drivers/cpufreq/cppc_cpufreq.c
b/drivers/cpufreq/cppc_cpufreq.c
index 8f950fe72765..a6cd95c3b474 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -422,12 +422,66 @@ static unsigned int
cppc_cpufreq_get_transition_delay_us(unsigned int cpu)
return cppc_get_transition_latency(cpu) / NSEC_PER_USEC;
}
+static bool efficiency_class_populated;
+static DEFINE_PER_CPU(unsigned int, efficiency_class);
+
+static int populate_efficiency_class(void)
+{
+ unsigned int min = UINT_MAX, max = 0, class;
+ struct acpi_madt_generic_interrupt *gicc;
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ gicc = acpi_cpu_get_madt_gicc(cpu);
+ if (!gicc)
+ return -ENODEV;
How can that happen if you made it here using ACPI?
This is effectively an extra check. This could be removed.
+
+ per_cpu(efficiency_class, cpu) = gicc->efficiency_class;
+ min = min_t(unsigned int, min, gicc->efficiency_class);
+ max = max_t(unsigned int, max, gicc->efficiency_class);
+ }
Why don't you use a temporary bitmap of 256 bits, tracking
the classes that are actually being used?
+
+ if (min == max) {
This would become (bitmap_weight(used_classes) <= 1). Then from
the same construct you know how many different classes you have.
You also have the min, max, and all the values in between.
+ pr_debug("Efficiency classes are all equal (=%d). "
+ "No EM registered", max);
+ return -EINVAL;
+ }
+
+ /*
+ * Squeeze efficiency class values on [0:#efficiency_class-1].
+ * Values are per spec in [0:255].
+ */
+ for (class = 0; class < 256; class++) {
+ unsigned int new_min, curr;
+
+ new_min = UINT_MAX;
+ for_each_possible_cpu(cpu) {
+ curr = per_cpu(efficiency_class, cpu);
+ if (curr == min)
+ per_cpu(efficiency_class, cpu) = class;
+ else if (curr > min)
+ new_min = min(new_min, curr);
+ }
+
+ if (new_min == UINT_MAX)
+ break;
+ min = new_min;
+ }
I find it really hard to reason about this because you are
dynamically rewriting the values you keep reevaluating.
How about something like this, which I find more readable:
DECLARE_BITMAP(used_classes, 256) = {};
int class, index, cpu;
for_each_possible_cpu(cpu) {
unsigned int ec;
ec = acpi_cpu_get_madt_gicc(cpu)->efficiency_class & 0xff;
bitmap_set(ec, &used_classes);
}
if (bitmap_weight(&used_classes, 256) <= 1)
return;
index = 0;
for_each_set_bit(class, &used_classes, 256) {
for_each_possible_cpu(cpu) {
if (acpi_cpu_get_madt_gicc(cpu)->efficiency_class == class)
per_cpu(efficiency_class, cpu) = index;
}
index++;
}
This is effectively much more readable. Thanks for the code snippet.
Regards,
Pierre
Thanks,
M.