linux-kernel - Re: [patch V4 24/41] x86/cpu: Provide cpu_init/parse

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Message-ID: <0eb56cda-511a-452d-7191-c19477276327@amd.com>
Date:   Wed, 20 Sep 2023 08:53:36 +0530
From:   K Prateek Nayak <kprateek.nayak@....com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Huang Rui <ray.huang@....com>, Juergen Gross <jgross@...e.com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Pu Wen <puwen@...on.cn>,
        Qiuxu Zhuo <qiuxu.zhuo@...el.com>,
        Sohil Mehta <sohil.mehta@...el.com>,
        Gautham Shenoy <gautham.shenoy@....com>
Subject: Re: [patch V4 24/41] x86/cpu: Provide cpu_init/parse_topology()

Hello Thomas,

On 9/19/2023 1:43 PM, Thomas Gleixner wrote:
> On Tue, Sep 19 2023 at 09:24, K Prateek Nayak wrote:
>> If possible, can you please elaborate on the "software perspective". Say
>> CPUID leaf 0x1f reports multiple tile, would the data access latency or
>> cache to cache latency see a noticeable difference?
>>
>> I would like to understand what the characteristics of a "Tile" are and
>> whether they are similar to AMD's CCX instances discoverable by AMD's
>> extended CPUID leaf 0x80000026. That way, in future, when the generic
>> topology is used by other subsystems, the data from "TOPO_TILE_DOMAIN"
>> can be used generically for both Intel and AMD.
> 
> I'm not convinced that this is possible. The meaning of these elements
> is unfortunately not cast in stone, so the association of e.g. cache
> boundaries is not necessarily static accross a larger range of CPU
> generations..
> 
> We really need to differentiate performance characteristic, hardware
> feature scope, power management scope etc. and create abstractions which
> are actually useful for kernel facilities and user space.
> 
> From a performance perspective it's irrelevant whether the scope is TILE
> or whatever. The information needed is the hierarchy, the performance
> characteristics of a segment in the hierarchy and the costs for cross
> segment access and cross segment migration.

That makes sense. Thank you for the detailed explanation.

> 
> For hardware features like perf the information needed is what the scope
> of a particular resource is. Again it's not necessarily useful to know
> what particular domain type it is.
> 
> Powermanagement does not care about the particual type either. It needs
> to know which scopes affect c-states, clock speeds ... to give the
> scheduler informtion of how to steer workloads in the most efficient way.
> 
> Thanks,
> 
>         tglx

--
Thanks and Regards,
Prateek