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

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Date: Wed, 6 Mar 2024 21:54:02 +0000
From: "Luck, Tony" <tony.luck@...el.com>
To: "Wieczor-Retman, Maciej" <maciej.wieczor-retman@...el.com>, "Yu, Fenghua"
	<fenghua.yu@...el.com>, "Chatre, Reinette" <reinette.chatre@...el.com>,
	"Shuah Khan" <shuah@...nel.org>, "james.morse@....com" <james.morse@....com>
CC: "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

> 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?

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?

-Tony