Re: [RFC][PATCH v1 3/3] cpufreq: intel_pstate: Set asymmetric CPU capacity on hybrid systems

Dietmar Eggemann
2024-05-24T12:43:58Z

On 25/04/2024 21:06, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> Make intel_pstate use the HWP_HIGHEST_PERF values from
> MSR_HWP_CAPABILITIES to set asymmetric CPU capacity information
> via the previously introduced arch_set_cpu_capacity() on hybrid
> systems without SMT.
>
> Setting asymmetric CPU capacity is generally necessary to allow the
> scheduler to compute task sizes in a consistent way across all CPUs
> in a system where they differ by capacity. That, in turn, should help
> to improve task placement and load balancing decisions. It is also
> necessary for the schedutil cpufreq governor to operate as expected
> on hybrid systems where tasks migrate between CPUs of different
> capacities.

[...]

For Arm64 we expose the cpu_capacity under:

/sys/devices/system/cpu/cpu*/cpu_capacity

Might be handy for X86 hybrid as well.

Code snippet copied from from drivers/base/arch_topology.c :

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 9e94b3f05a57..c445e5d1efc8 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -3746,5 +3746,49 @@ static int __init intel_pstate_setup(char *str)
}
early_param("intel_pstate", intel_pstate_setup);

+static ssize_t cpu_capacity_show(struct device *dev,
+ struct device_attribute *attr,
+ char *buf)
+{
+ struct cpu *cpu = container_of(dev, struct cpu, dev);
+
+ return sysfs_emit(buf, "%lu\n", arch_scale_cpu_capacity(cpu->dev.id));
+}
+
+static DEVICE_ATTR_RO(cpu_capacity);
+
+static int cpu_capacity_sysctl_add(unsigned int cpu)
+{
+ struct device *cpu_dev = get_cpu_device(cpu);
+
+ if (!cpu_dev)
+ return -ENOENT;
+
+ device_create_file(cpu_dev, &dev_attr_cpu_capacity);
+
+ return 0;
+}
+
+static int cpu_capacity_sysctl_remove(unsigned int cpu)
+{
+ struct device *cpu_dev = get_cpu_device(cpu);
+
+ if (!cpu_dev)
+ return -ENOENT;
+
+ device_remove_file(cpu_dev, &dev_attr_cpu_capacity);
+
+ return 0;
+}
+
+static int register_cpu_capacity_sysctl(void)
+{
+ cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "topology/cpu-capacity",
+ cpu_capacity_sysctl_add, cpu_capacity_sysctl_remove);
+
+ return 0;
+}
+subsys_initcall(register_cpu_capacity_sysctl);
+ …