linux-kernel - RE: [PATCH v16 0/9] Add support for Sub-NUMA cluster (SNC) systems

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Message-ID: <SJ1PR11MB6083B59F32F33C0FFBFCA1BCFC2C2@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date: Tue, 19 Mar 2024 20:31:42 +0000
From: "Luck, Tony" <tony.luck@...el.com>
To: "Chatre, Reinette" <reinette.chatre@...el.com>, "Yu, Fenghua"
	<fenghua.yu@...el.com>, Peter Newman <peternewman@...gle.com>, "Jonathan
 Corbet" <corbet@....net>, Shuah Khan <skhan@...uxfoundation.org>,
	"x86@...nel.org" <x86@...nel.org>
CC: Shaopeng Tan <tan.shaopeng@...itsu.com>, James Morse
	<james.morse@....com>, Jamie Iles <quic_jiles@...cinc.com>, Babu Moger
	<babu.moger@....com>, Randy Dunlap <rdunlap@...radead.org>, Drew Fustini
	<dfustini@...libre.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>, "patches@...ts.linux.dev"
	<patches@...ts.linux.dev>, "Wieczor-Retman, Maciej"
	<maciej.wieczor-retman@...el.com>
Subject: RE: [PATCH v16 0/9] Add support for Sub-NUMA cluster (SNC) systems

> I agree to drop patch #9.
>
> The core support for SNC continue to look good to me (I just had a few nitpicks).
>
> What remains is the user interface that continues to gather opinions [3]. These new
> discussions were prompted by user space needing a way to determine if resctrl supports
> SNC. This started by using the "size" file but thinking about it more user space could
> also look at whether the number of L3 control domains are different from the number
> of L3 monitoring domains? I am adding Maciej for his opinion (please also include him
> in future versions of this series).

Implementing the revised user interface will make significant changes to the
patches needed to support SNC. Working on them now.

Minor stuff is the that "size" file won't need to change (because summing across
all SNC domains mean that all of the cache is counted in mon_data/mon_L3_xx/llc_occupancy.
The new mon_data/mon_L3_xx/mon_NODE_xxllc_occupancy files are the ones limited
to 1/snc_ways of L3 capacity.

Major stuff is that we now need both the L3-scoped domain list as well as the NODE
scoped domain list. So no longer just changing the "mon_scope" field in the L3
rdt_resource from one value to another.

I'll maybe have a draft set of patches in a day or two.

> Apart from the user space requirement to know if SNC is supported by resctrl there
> is also the interface with which user space obtains the monitoring data.
> James highlighted [1] that the interface used in this series uses existing files to
> represent different content, and can thus be considered as "broken". It is not obvious
> to me how to "fix" this. Should we continue to explore interfaces like [2] that
> attempts to add SNC support into resctrl or should the message continue to be
> that SNC "plays havoc with the RDT monitoring features" and users wanting to use
> SNC and RDT at the same time are expected to adapt to the peculiar interface ...
> or is the preference that after this series "SNC and RDT are compatible" and
> thus presented with an intuitive interface?
>
> Reinette
>
> [1] https://lore.kernel.org/lkml/88430722-67b3-4f7d-8db2-95ee52b6f0b0@arm.com/
> [2] https://lore.kernel.org/lkml/SJ1PR11MB608309F47C00F964E16205D6FC2D2@SJ1PR11MB6083.namprd11.prod.outlook.com/
> [3] https://lore.kernel.org/lkml/SJ1PR11MB608310C72D7189C139EA6302FC212@SJ1PR11MB6083.namprd11.prod.outlook.com/