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

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Date:   Fri, 13 May 2022 12:03:12 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Qing Wang <wangqing@...o.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        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 Fri, May 13, 2022 at 12:42:00PM +0200, Dietmar Eggemann wrote:
> 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.

Not sure if that is the correct representation of h/w cluster on those
platforms, but if they want to misguide OS with the f/w(DT in this case)
well that's their choice.

The main point is we need to get the exact h/w topology information and
then we can decide how to present the below masks as required by the
scheduler for its sched domains.

> 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.
>

I am not sure on the above point. Even with ACPI PPTT we are just setting
cluster_id based on first or leaf level of the clusters ignoring the
nesting ATM. And that's exactly what I am trying to get with this series[1]

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

Sure, it is already supported IIUC by cpu_coregroup_mask in arch_topology.c
We just need to make sure llc_id is set correctly in case of DT. Now if
you are saying llc_sibling is not what you need but something else, then
we may need to add that new mask and update cpu_coregroup_mask to choose
that based on certain condition which I believe is bit complicated.

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

Will grouping of CPUs into cluster they belong not good enough for CLS ?
I thought that should suffice based on what we have in cpu_clustergroup_mask
(i.e. cluster sibling mask)

--
Regards,
Sudeep