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

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Date: Thu, 7 Mar 2024 10:25:45 +0100
From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
To: "Luck, Tony" <tony.luck@...el.com>
CC: "Yu, Fenghua" <fenghua.yu@...el.com>, "Chatre, Reinette"
	<reinette.chatre@...el.com>, Shuah Khan <shuah@...nel.org>,
	"james.morse@....com" <james.morse@....com>, "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

On 2024-03-06 at 21:54:02 +0000, Luck, Tony wrote:
>> 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.
>
>Early versions of the kernel SNC patch series added an info/l3_MON/snc_ways
>file to provide this information. I was talked out of it then:
>
>https://lore.kernel.org/all/f0841866-315b-4727-0a6c-ec60d22ca29c@arm.com/
>
>But that discussion didn't consider the question you discuss here: "Does this
>instance of the kernel support SNC?"
>
>So you have a clever solution. But it seems like a roundabout way for
>an application to discover whether the kernel has configured resctrl for
>SNC mode.
>
>Should the kernel provide an info/ file listing the SNC configuration?

I suppose it would be a benefit for other numa aware applications to have an
easy access to this kernel support information.

>
>If so, what should it be named? "snc_ways" as a kernel variable was
>later replaced by "snc_nodes_per_l3_cache". Is that a good filename?

"snc_nodes_per_l3_cache" seems okay to me.

And I understand that the file content would show SNC mode and the presence or
absence of this file would tell if kernel supports SNC?

>
>-Tony

-- 
Kind regards
Maciej Wieczór-Retman