linux-kernel - Re: [PATCH v12 06/10] fs/resctrl: Add user interface to enable/disable io

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Message-ID: <312511a1-ee7c-47da-b351-ee7d9adfe306@amd.com>
Date: Thu, 20 Nov 2025 12:35:26 -0600
From: Babu Moger <bmoger@....com>
To: Babu Moger <babu.moger@....com>, tony.luck@...el.com,
 reinette.chatre@...el.com, tglx@...utronix.de, mingo@...hat.com,
 bp@...en8.de, dave.hansen@...ux.intel.com
Cc: corbet@....net, Dave.Martin@....com, james.morse@....com, x86@...nel.org,
 hpa@...or.com, akpm@...ux-foundation.org, paulmck@...nel.org,
 rdunlap@...radead.org, pmladek@...e.com, kees@...nel.org, arnd@...db.de,
 fvdl@...gle.com, seanjc@...gle.com, pawan.kumar.gupta@...ux.intel.com,
 xin@...or.com, thomas.lendacky@....com, sohil.mehta@...el.com,
 jarkko@...nel.org, chang.seok.bae@...el.com, ebiggers@...gle.com,
 elena.reshetova@...el.com, ak@...ux.intel.com, mario.limonciello@....com,
 perry.yuan@....com, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 peternewman@...gle.com, feng.tang@...ux.alibaba.com
Subject: Re: [PATCH v12 06/10] fs/resctrl: Add user interface to
 enable/disable io_alloc feature

Hi Reinette,

On 11/12/25 18:57, Babu Moger wrote:
> AMD's SDCIAE forces all SDCI lines to be placed into the L3 cache portions
> identified by the highest-supported L3_MASK_n register, where n is the
> maximum supported CLOSID.
>
> To support AMD's SDCIAE, when io_alloc resctrl feature is enabled, reserve
> the highest CLOSID exclusively for I/O allocation traffic making it no
> longer available for general CPU cache allocation.
>
> Introduce user interface to enable/disable io_alloc feature and encourage
> users to enable io_alloc only when running workloads that can benefit from
> this functionality. On enable, initialize the io_alloc CLOSID with all
> usable CBMs across all the domains.
>
> Since CLOSIDs are managed by resctrl fs, it is least invasive to make
> "io_alloc is supported by maximum supported CLOSID" part of the initial
> resctrl fs support for io_alloc. Take care to minimally (only in error
> messages) expose this use of CLOSID for io_alloc to user space so that this
> is not required from other architectures that may support io_alloc
> differently in the future.
>
> When resctrl is mounted with "-o cdp" to enable code/data prioritization,
> there are two L3 resources that can support I/O allocation: L3CODE and
> L3DATA.  From resctrl fs perspective the two resources share a CLOSID and
> the architecture's available CLOSID are halved to support this.  The
> architecture's underlying CLOSID used by SDCIAE when CDP is enabled is the
> CLOSID associated with the CDP_CODE resource, but from resctrl's perspective
> there is only one CLOSID for both CDP_CODE and CDP_DATA. CDP_DATA is thus
> not usable for general (CPU) cache allocation nor I/O allocation. Keep the
> CDP_CODE and CDP_DATA I/O alloc status in sync to avoid any confusion to
> user space. That is, enabling io_alloc on CDP_CODE does so on CDP_DATA and
> vice-versa, and keep the I/O allocation CBMs of CDP_CODE and CDP_DATA in
> sync.
>
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---
> v12: Minor format fix in resctrl.rst.
>       Added text about writable files in the info section.
>       Removed Reviewed-by tag from Reinette as there are some new text changes from Dave.

Only this patch needs "Reviewed-by" tag.

Gentle reminder with upcoming holidays.

Hoping this series will be queued for next merge window.

Thanks

Babu