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

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Message-ID: <SJ1PR11MB60832DAD58E864F99A16FCC4FC202@SJ1PR11MB6083.namprd11.prod.outlook.com>
Date: Thu, 7 Mar 2024 17:57:12 +0000
From: "Luck, Tony" <tony.luck@...el.com>
To: "Chatre, Reinette" <reinette.chatre@...el.com>, "Wieczor-Retman, Maciej"
	<maciej.wieczor-retman@...el.com>
CC: "Yu, Fenghua" <fenghua.yu@...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

> > SNC2 enabled:
> >
> > $ cat /sys/fs/resctrl/info/L3_mon/ snc_nodes_per_l3_cache
> > 2
> >
>
> This would be useful. I believe "SNC" is architecture specific?
> What if the file always exists and is named "nodes_per_l3_cache"?
>
> I assume that the internals of handling more nodes per L3 cache should
> be hidden from user space and it should not be necessary for user space
> to know if this is because of SNC or potentially some other mechanism on
> another platform?
>
> I think that may reduce fragmentation of resctrl .... not having
> resctrl look so different on different architectures but maintains
> the promise of a generic interface.
>
> I am not sure if this is specific to monitoring though,
> why not host file in /sys/fs/resctrl/info/L3 ?

Reinette,

On the name change - sure. It doesn't need the "snc_" prefix.

The Intel implementation of SNC has far more effect on monitoring
than on control. The user can read separate cache occupancy and
memory bandwidth values for each SNC node. But cache allocation
bitmasks and memory throttling still have a single control point for
each L3 cache instance, not for each node. There are still some
impacts on control, e.g. each bit in a CAT bitmask represents
less actual space in the L3 cache.

Maybe move it to the top level of the info/ directory:

$ cat /sys/fs/resctrl/info/nodes_per_l3_cache
3

-Tony