linux-kernel - Re: [PATCH 0/9] Add CPU-type to topology

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Date: Thu, 20 Jun 2024 17:22:59 +0200
From: Brice Goglin <brice.goglin@...il.com>
To: Dave Hansen <dave.hansen@...el.com>,
 srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, daniel.sneddon@...ux.intel.com, tony.luck@...el.com,
 linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
 linux-perf-users@...r.kernel.org, Josh Poimboeuf <jpoimboe@...nel.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
 Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
 "Liang, Kan" <kan.liang@...ux.intel.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH 0/9] Add CPU-type to topology

Le 20/06/2024 à 17:06, Dave Hansen a écrit :

> On 6/19/24 14:25, Brice Goglin wrote:
>> Good point. From this patch series, I understand that the current kernel
>> side doesn't care about these different E-cores. However it might be
>> good to expose them as different cpu-types (or better name) to userspace ?
>>
>> Something like type 0 = P-core, 1 = normal E-core, 2 = low power E-core ?
> The first priority here is getting the kernel to comprehend these types
> for architectural purposes: when there are functional differences
> between the cores.
>
> Let's get that in place, first.  Then we can discuss the possibility of
> new ABI in the area.

Agreed.

> Did the ARM folks ever do a sysfs ABI for big.LITTLE?  I don't see
> anything obvious in Documentation/ABI/testing/sysfs-devices-system-cpu.

As far as I know, they only have the "capacity" field in sysfs cpu files
that reports a higher number for the equivalent of P-core:

 From testing/sysfs-devices-system-cpu:

What:           /sys/devices/system/cpu/cpuX/cpu_capacity
Date:           December 2016
Contact:        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Description:    information about CPUs heterogeneity.

                 cpu_capacity: capacity of cpuX.

I don't know how it's calculated but I've never seen it report something wrong.
On Android/ARM phones, big cores usually have 1024 and small cores something
between 400 and 500.
Where there are 3 types of cores, they report 1024, 500-800 about ~250.

Brice