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Date: Fri, 24 May 2024 14:43:41 +0200
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Subject: Re: [RFC][PATCH v1 3/3] cpufreq: intel_pstate: Set asymmetric CPU
 capacity on hybrid systems
To: "Rafael J. Wysocki" <rjw@rjwysocki.net>, x86 Maintainers <x86@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Linux PM <linux-pm@vger.kernel.org>,
 Thomas Gleixner <tglx@linutronix.de>, Peter Zijlstra <peterz@infradead.org>,
 Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
 "Rafael J. Wysocki" <rafael@kernel.org>,
 Ricardo Neri <ricardo.neri@intel.com>, Tim Chen <tim.c.chen@intel.com>
References: <7663799.EvYhyI6sBW@kreacher> <1799046.VLH7GnMWUR@kreacher>
From: Dietmar Eggemann <dietmar.eggemann@arm.com>
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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);
+

[...]