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

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Date: Thu, 30 May 2024 16:07:34 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>,
	<fenghua.yu@...el.com>, <shuah@...nel.org>
CC: <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

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].

> 
> 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/