linux-kernel - Re: [RFC PATCH] arch_topology: Use llc_id instead of package

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Message-ID: <afafbb0c-5279-bee8-1ef4-434733e2a552@arm.com>
Date:   Fri, 13 May 2022 12:42:00 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Qing Wang <wangqing@...o.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] arch_topology: Use llc_id instead of package_id

On 13/05/2022 11:03, Sudeep Holla wrote:
> On Fri, May 13, 2022 at 10:34:00AM +0200, Dietmar Eggemann wrote:

[...]

>> @@ -527,7 +528,8 @@ static int __init parse_core(struct device_node *core, int package_id,
>>  			return -EINVAL;
>>  		}
>>  
>> -		cpu_topology[cpu].package_id = package_id;
>> +		cpu_topology[cpu].package_id = 0;
> 
> While the above looks good and matches with what I am attempting to do
> as well ...
> 
>> +		cpu_topology[cpu].llc_id = llc_id;
> 
> This looks wrong for simple reason that this is derived incorrectly from
> the cpu-map while there is no guarantee that it matches the last level
> cache ID on the system as we didn't parse the cache topology for this.
> So I disagree with this change as it might conflict with the actual and
> correct llc_id.

It might not match the LLC, that's true. Something we have already today
in Android for DynamIQ clusters with big/Little. People using 1. level
clusters to group CPUs according to uArch.

My point is we manage to get:

SMT - cpu_smt_mask()
CLS - cpu_clustergroup_mask()
MC  - cpu_coregroup_mask()
DIE - cpu_cpu_mask()

covered in ACPI with the cpu_topology[] structure and if we want CLS on
DT we have to save cluster_id for the 2. level (DT) cluster.

And that's why I proposed to (ab)use llc_id to form the MC mask.

I'm not currently aware of another solution to get CLS somehow elegantly
into a DT system.