linux-kernel - Re: [PATCH 4/4] selftests/resctrl: Adjust SNC support messages

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Message-ID: <eacdc287-24bd-4137-85c8-df055cfd78b1@arm.com>
Date: Fri, 8 Mar 2024 18:06:45 +0000
From: James Morse <james.morse@....com>
To: Tony Luck <tony.luck@...el.com>,
 Reinette Chatre <reinette.chatre@...el.com>
Cc: "Wieczor-Retman, Maciej" <maciej.wieczor-retman@...el.com>,
 "Yu, Fenghua" <fenghua.yu@...el.com>, Shuah Khan <shuah@...nel.org>,
 "ilpo.jarvinen@...ux.intel.com" <ilpo.jarvinen@...ux.intel.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH 4/4] selftests/resctrl: Adjust SNC support messages

Hi guys,

On 07/03/2024 23:16, Tony Luck wrote:
> On Thu, Mar 07, 2024 at 02:39:08PM -0800, Reinette Chatre wrote:
>> Thank you for the example. I find that significantly easier to
>> understand than a single number in a generic "nodes_per_l3_cache".
>> Especially with potential confusion surrounding inconsistent "nodes"
>> between allocation and monitoring. 
>>
>> How about domain_cpu_list and domain_cpu_map ?

> Like this (my test system doesn't have SNC, so all domains are the same):
> 
> $ cd /sys/fs/resctrl/info/
> $ grep . */domain*
> L3/domain_cpu_list:0: 0-35,72-107
> L3/domain_cpu_list:1: 36-71,108-143
> L3/domain_cpu_map:0: 0000,00000fff,ffffff00,0000000f,ffffffff
> L3/domain_cpu_map:1: ffff,fffff000,000000ff,fffffff0,00000000
> L3_MON/domain_cpu_list:0: 0-35,72-107
> L3_MON/domain_cpu_list:1: 36-71,108-143
> L3_MON/domain_cpu_map:0: 0000,00000fff,ffffff00,0000000f,ffffffff
> L3_MON/domain_cpu_map:1: ffff,fffff000,000000ff,fffffff0,00000000
> MB/domain_cpu_list:0: 0-35,72-107
> MB/domain_cpu_list:1: 36-71,108-143
> MB/domain_cpu_map:0: 0000,00000fff,ffffff00,0000000f,ffffffff
> MB/domain_cpu_map:1: ffff,fffff000,000000ff,fffffff0,00000000

This duplicates the information in /sys/devices/system/cpu/cpuX/cache/indexY ... is this
really because that information is, er, wrong on SNC systems. Is it possible to fix that?

>From Tony's earlier description of how SNC changes things, the MB controls remain
per-socket. To me it feels less invasive to fix the definition of L3 on these platforms to
describe how it behaves (assuming that is possible), and define a new 'MB' that is NUMA
scoped.
This direction of redefining L3 means /sys/fs/resctrl and /sys/devices have different
views of 'the' cache hierarchy.

(I also think that this be over the threshold on 'funny machines look funny' - but I bet
someone builds an arm machine that looks like this too!)


Thanks,

James