linux-kernel - Re: [PATCH v2 3/3] x86/resctrl: Add "*" shorthand to set minimum io

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Message-ID: <83cd3fe0-2dc6-4456-b845-4072b7ecb2a2@intel.com>
Date: Fri, 19 Dec 2025 10:28:32 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: Aaron Tomlin <atomlin@...mlin.com>, "Luck, Tony" <tony.luck@...el.com>
CC: "Moger, Babu" <bmoger@....com>, "Dave.Martin@....com"
	<Dave.Martin@....com>, "james.morse@....com" <james.morse@....com>,
	"babu.moger@....com" <babu.moger@....com>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "mingo@...hat.com" <mingo@...hat.com>, "bp@...en8.de"
	<bp@...en8.de>, "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"sean@...e.io" <sean@...e.io>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] x86/resctrl: Add "*" shorthand to set minimum
 io_alloc CBM for all domains

Hi Aaron and Tony,

On 12/18/25 4:44 PM, Aaron Tomlin wrote:
> On Thu, Dec 18, 2025 at 02:59:59PM -0800, Reinette Chatre wrote:
>> If I remember correctly the idea was to limit this feature to io_alloc to
>> avoid needing to deal with L2 asymmetric domains [1].
> 
> Hi Reinette,
> 
> You are quite right; I am in complete agreement with your assessment. The
> primary intention behind limiting the scope to io_alloc was indeed to avoid
> the complexities associated with L2 asymmetric domains.
> 
> Are we all in alignment to focus this feature entirely on io_alloc for the
> time being? If so, I will be pleased to prepare a follow-up series that

>From my side I am ok to limit this to io_alloc. Of course, this does not prevent
cache allocation from supporting this syntax in the future.

Tony: did you perhaps imply with examples in [2] that '*' only be supported by
L3 and MB, not L2? Can it be guaranteed that L3 will never be asymmetric? Not that
it is a blocker though, as discussed earlier there are ways [3] to support
'*' when domains may be asymmetric. That proposal is only reasonable if considering the
feature as "let user set same CBM on all domains" that just happens to support the "reset
to min" use case for L3 io_alloc. I assume even on asymmetric domains the min would be the
same? If there is indeed a requirement to support "reset to defaults" for general cache
allocation then this feature would not work for asymmetric domains as you highlighted in [4].
Although, a "reset to defaults" for cache allocation use case may be better handled by
removing and recreating the resource group since the defaults will take into account
any exclusive allocations?

Reinette

> reflects this consensus once our wider discussion has concluded.
> 
>>
>> [1] https://lore.kernel.org/lkml/SJ1PR11MB60833A27A1B8057CDDFB1B2BFCCFA@SJ1PR11MB6083.namprd11.prod.outlook.com/
[2] https://lore.kernel.org/all/SJ1PR11MB6083CCA2A7904E459B1AA35DFCA8A@SJ1PR11MB6083.namprd11.prod.outlook.com/
[3] https://lore.kernel.org/lkml/f4a043d2-9cb0-41c9-a45d-31f96fd007d5@amd.com/
[4] https://lore.kernel.org/lkml/SJ1PR11MB60836AB4270419338FBB4D1EFCCEA@SJ1PR11MB6083.namprd11.prod.outlook.com/