linux-kernel - Re: [EXT] Re: [RFC 00/12] ARM: MPAM: add support for priority partitioning control

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Date: Thu, 11 Jan 2024 12:56:34 -0800
From: Peter Newman <peternewman@...gle.com>
To: Amit Singh Tomar <amitsinght@...vell.com>
Cc: Reinette Chatre <reinette.chatre@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, 
	"fenghua.yu@...el.com" <fenghua.yu@...el.com>, "james.morse@....com" <james.morse@....com>, 
	George Cherian <gcherian@...vell.com>, "robh@...nel.org" <robh@...nel.org>, 
	"Luck, Tony" <tony.luck@...el.com>
Subject: Re: [EXT] Re: [RFC 00/12] ARM: MPAM: add support for priority
 partitioning control

Hi Amit,

On Thu, Aug 24, 2023 at 1:52 AM Amit Singh Tomar <amitsinght@...vell.com> wrote:

> 2) Second approach that we discussed internally is to have schemata for CPOR, and PPART separated by new line as mentioned/suggested by Peter, But it may require to tweak
>    the ARM MPAM device driver a bit. It was kind of toss-up between 2nd and 3nd approach :), and we went with the 3rd one.
>
>    L3:0=XXXX
>    L3:0=PPART=X
>
>    Will look into it again.

I've been looking into the structure of the MPAM driver to understand
the difficulties here.

It seems the challenge with DSPRI is trying to stuff two different
control schema (partitioning, prioritization) into the L3
rdt_resource. The rdt_resource is still a mix of a hardware component
and user interface (schema line), which leads to  the
__resource_props_mismatch() function[1], which makes an arbitrary
choice (driven by resource index order) of which feature should be the
single control presented for each of the rdt_resources, the properties
of which the fields of its rdt_resource entry should describe.

It only seemed to work out for CDP emulation because the properties of
the two schema (L3CODE, L3DATA) for the L3 resource have the same CBM
properties.

My opinion is that the rdt_resource needs to be removed from
fs/resctrl and replaced with a structure to represent a control schema
and another to represent a monitor so that we don't find ourselves
unable to enumerate controls or monitors because a control or monitor
from the same hardware component has already been enumerated.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/tree/drivers/platform/mpam/mpam_devices.c?h=mpam/snapshot/v6.7-rc2#n1810

-Peter