Message-ID: <bcc8e6d2-8e91-4e91-9dab-f1c00f99c408@amd.com>
Date: Mon, 10 Nov 2025 10:34:48 -0600
From: Babu Moger <bmoger@....com>
To: Aaron Tomlin <atomlin@...mlin.com>
Cc: tony.luck@...el.com, reinette.chatre@...el.com, Dave.Martin@....com,
 james.morse@....com, babu.moger@....com, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/resctrl: Add io_alloc_min_cbm_all interface for
 CBM reset

Hi Aaron,

On 11/7/25 18:19, Aaron Tomlin wrote:
> On Fri, Nov 07, 2025 at 05:08:03PM -0600, Moger, Babu wrote:
>> No, I don’t agree with introducing this new interface.
>>
>> These settings are intended to be modified only by system administrators,
>> not general users.
>>
>> Administrators are already expected to have complete knowledge of the
>> hardware and corresponding configuration details. They also typically rely
>> on benchmark data to determine appropriate settings.
>>
>> Additionally, this approach is not consistent with our existing CBM setting
>> model.
> Hi Babu,
>
> I fully appreciate your point that system administrators are expected to
> have complete knowledge of the hardware configuration. I believe my initial
> motivation may have been unclear, so let me clarify the intention behind
> "io_alloc_min_cbm_all".
>
> The intention of this new interface is not to hide lower-level details from
> the administrator, but rather to simplify a specific, high-overhead
> administration task and prevent human error, which I encountered during
> testing of your series.
>
>
>
> By default, it is my understanding that when I/O allocation CBM is enabled,
> all applicable shared L3 resources typically have their CBM bits set (e.g.,
> ffff). If an administrator wishes to revert to a clean baseline before
> applying a subset of dedicated bits, the manual process requires them to
> not only have knowledge of every single Domain ID
> applicable to the shared L3 resource but manually construct a long,
> error-prone input string.
>
> The objective of "io_alloc_min_cbm_all" is to provide a quick and accurate
> medium to revert this topology-dependent default configuration to the
> minimal supported state, thereby avoiding possible miscalculation or
> omission of a Domain ID in the CBM string. As shown in the usage flow
> below, the new interface drastically reduces the complexity of setting up a
> specific configuration:
>
> # cat /sys/fs/resctrl/info/L3/io_alloc_cbm
> 0=ffff;2=ffff;4=ffff;6=ffff;8=ffff;10=ffff;12=ffff;14=ffff;16=ffff;18=ffff;20=ffff;22=ffff;24=ffff;26=ffff;28=ffff;30=ffff;32=ffff;34=ffff;36=ffff;38=ffff;40=ffff;42=ffff;44=ffff;46=ffff;48=ffff;50=ffff;52=ffff
>
> # echo 0=0;2=0;4=0;6=4;8=4;10=0;12=0;14=0;16=ffff;18=ffff;20=0;22=0;24=0;26=0;28=0;30=0;32=0;34=4;36=0;38=0;40=0;42=0;44=0;46=0;48=0;50=0;52=0 > /sys/fs/resctrl/info/L3/io_alloc_cbm


Does the following option work for you?

# echo "*=0" > /sys/fs/resctrl/info/L3/io_alloc_cbm


Here,|*| represents all domains.


This functionality was introduced when adding support for the*"mbm_event" assign mode* (see [1]).
[1]https://lore.kernel.org/lkml/b894ad853e6757d40da1469bf9fca4c64684df65.1757108044.git.babu.moger@amd.com/ <https://lore.kernel.org/lkml/b894ad853e6757d40da1469bf9fca4c64684df65.1757108044.git.babu.moger@amd.com/> <https://lore.kernel.org/lkml/b894ad853e6757d40da1469bf9fca4c64684df65.1757108044.git.babu.moger@amd.com/>
Also, this needs to be done for all the settings like L3, MBA also SMBA.

# echo "L3:*=f" > /sys/fs/resctrl/schemata

# echo "MB:*=128" > /sys/fs/resctrl/schemata

I’d like to hear from Reinette and Tony if this seems like an acceptable 
approach.

Thanks

Babu

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