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

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Date: Thu, 27 Jun 2024 05:55:47 -0700
From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To: Brice Goglin <brice.goglin@...il.com>
Cc: Dave Hansen <dave.hansen@...el.com>,
	srinivas pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	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>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH 0/9] Add CPU-type to topology

On Thu, Jun 20, 2024 at 05:22:59PM +0200, Brice Goglin wrote:
> 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.

I *think* this information comes from firmware tables, but you cannot reliably
infer CPU types from capacity (e.g., Intel's Ecores vs Low Power Ecores).

Also, it is not useful to identify "performance" vs "efficient" CPUs.