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

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Message-ID: <aUW4s-HG8XqCbuMH@agluck-desk3>
Date: Fri, 19 Dec 2025 12:42:27 -0800
From: "Luck, Tony" <tony.luck@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>
CC: Aaron Tomlin <atomlin@...mlin.com>, "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

On Fri, Dec 19, 2025 at 10:28:32AM -0800, Reinette Chatre wrote:
> 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

I'd forgotten the L2 asymmetry issue. If we wanted to enable "*" more
generally, then resctrl would have to limit it to symmetric resources
or to allow setting values that work for all domains in an asymmetric
resource).  But that seems more complexity in the kernel for something
than can easily be handled in user space. E.g. to reset L3 to ffff

# sed -n -e '/L3:/s/\(=[0-9a-f][0-9a-f]*\)/=ffff/gp' schemata > schemata

> '*' 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?

Removing the directory would free the RMID and allocate a new one when you
recreated it. Losing cache occupancy information completely, and disturbing
memory bandwidth monitoring. Also leaving the user to hunt down tasks
that were reassigned to the default CLOS and reassign them. So too many
side effects.
> 
> 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/

-Tony