linux-kernel - Re: [PATCH v2 2/2] selftests/resctrl: Adjust SNC support messages

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Date: Fri, 31 May 2024 08:39:47 +0200
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>
CC: <fenghua.yu@...el.com>, <shuah@...nel.org>,
	<linux-kselftest@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<ilpo.jarvinen@...ux.intel.com>, <tony.luck@...el.com>
Subject: Re: [PATCH v2 2/2] selftests/resctrl: Adjust SNC support messages

Hello!

On 2024-05-30 at 16:07:34 -0700, Reinette Chatre wrote:
>Hi Maciej,
>
>On 5/15/24 4:18 AM, Maciej Wieczor-Retman wrote:
>> Resctrl selftest prints a message on test failure that Sub-Numa
>> Clustering (SNC) could be enabled and points the user to check theirs BIOS
>> settings. No actual check is performed before printing that message so
>> it is not very accurate in pinpointing a problem.
>> 
>> Figuring out if SNC is enabled is only one part of the problem, the
>> other being whether the kernel supports it. As there is no easy
>> interface that simply states SNC support in the kernel one can find that
>> information by comparing L3 cache sizes from different sources. Cache
>> size reported by /sys/devices/system/node/node0/cpu0/cache/index3/size
>> will always show the full cache size even if it's split by enabled SNC.
>> On the other hand /sys/fs/resctrl/size has information about L3 size,
>> that with kernel support is adjusted for enabled SNC.
>> 
>> Add a function to find a cache size from /sys/fs/resctrl/size since
>> finding that information from the other source is already implemented.
>> 
>> Add a function that compares the two cache sizes and use it to make the
>> SNC support message more meaningful.
>
>Please note that the new version of SNC kernel support ([1]) that this
>series is based on no longer adjusts the cache size. Detecting kernel support
>for SNC (if the new solution is accepted) can be done with the test for the
>existence of the files Tony mentioned in [2].

Thank you for your comments on both patches, I don't know how I missed this
fact. I'll revise my patches accordingly.

>
>> 
>> Add the SNC support message just after MBA's check_results() since MBA
>> shares code with MBM and also can suffer from enabled SNC if there is no
>> support in the kernel.
>> 
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
>> ---
>
>Reinette
>
>[1] https://lore.kernel.org/all/20240503203325.21512-1-tony.luck@intel.com/
>[2] https://lore.kernel.org/lkml/SJ1PR11MB6083320F30DBCBB59574F0BDFCEC2@SJ1PR11MB6083.namprd11.prod.outlook.com/
>

-- 
Kind regards
Maciej Wieczór-Retman