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

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Message-ID: <81f357c1-aab5-45db-adfe-821efdbd0ae6@intel.com>
Date: Thu, 20 Nov 2025 20:41:23 -0800
From: Reinette Chatre <reinette.chatre@...el.com>
To: Babu Moger <babu.moger@....com>, <tony.luck@...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 Babu,

On 11/12/25 4:57 PM, 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>
> ---

Thank you.

Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Reinette