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

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Date: Wed, 20 Mar 2024 16:21:10 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>
CC: Tony Luck <tony.luck@...el.com>, Fenghua Yu <fenghua.yu@...el.com>, "Peter
 Newman" <peternewman@...gle.com>, Jonathan Corbet <corbet@....net>, "Shuah
 Khan" <skhan@...uxfoundation.org>, <x86@...nel.org>, 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-doc@...r.kernel.org>,
	<patches@...ts.linux.dev>
Subject: Re: [PATCH v16 0/9] Add support for Sub-NUMA cluster (SNC) systems

Hi Reinette,

On 2024-03-19 at 10:51:14 -0700, Reinette Chatre wrote:
>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). 

By this do you mean comparing the contents of main "schemata" file with the
number of mon_L3_* files?

>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?

I kind of liked this idea [1]. Hiding SNC related information behind some not
obvious text parsing and size comparisons might eliminate any ease of use for
userspace applications. But I agree with you [2] that it's hard to predict the
future for this interface and any potential problems with setting up this
file structure.

[1] https://lore.kernel.org/all/SJ1PR11MB608309F47C00F964E16205D6FC2D2@SJ1PR11MB6083.namprd11.prod.outlook.com/
[2] https://lore.kernel.org/all/7f15a700-f23a-48f9-b335-13ea1735ad84@intel.com/

-- 
Kind regards
Maciej Wieczór-Retman