linux-kernel - Re: [PATCH v7 07/10] fs/resctrl: Add user interface to enable/disable io

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Message-ID: <fe6eb3bc-03c9-40a1-9bab-35d755548246@amd.com>
Date: Mon, 4 Aug 2025 12:14:43 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, "Moger, Babu"
 <bmoger@....com>, corbet@....net, tony.luck@...el.com, Dave.Martin@....com,
 james.morse@....com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com
Cc: x86@...nel.org, hpa@...or.com, akpm@...ux-foundation.org,
 paulmck@...nel.org, rostedt@...dmis.org, Neeraj.Upadhyay@....com,
 david@...hat.com, arnd@...db.de, fvdl@...gle.com, seanjc@...gle.com,
 thomas.lendacky@....com, pawan.kumar.gupta@...ux.intel.com,
 yosry.ahmed@...ux.dev, sohil.mehta@...el.com, xin@...or.com,
 kai.huang@...el.com, xiaoyao.li@...el.com, peterz@...radead.org,
 me@...aill.net, mario.limonciello@....com, xin3.li@...el.com,
 ebiggers@...gle.com, ak@...ux.intel.com, chang.seok.bae@...el.com,
 andrew.cooper3@...rix.com, perry.yuan@....com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 07/10] fs/resctrl: Add user interface to enable/disable
 io_alloc feature

Hi Reinette,

On 8/4/25 11:07, Reinette Chatre wrote:
> Hi Babu,
> 
> On 7/31/25 3:52 PM, Moger, Babu wrote:
>> Hi Reinette,
>>
>> On 7/21/2025 6:40 PM, Reinette Chatre wrote:
>>> Hi Babu,
>>>
>>> On 7/10/25 10:16 AM, Babu Moger wrote:
>>>> "io_alloc" feature in resctrl enables direct insertion of data from I/O
>>>> devices into the cache.
>>>>
>>>> Introduce user interface to enable/disable io_alloc feature.
>>>
>>> I think it is worth a mention *why* a user may want to disable this feature and
>>> why is not just always enabled. Here it can be highlighted that this feature
>>> may take resources (CLOSID) away from general (CPU) cache allocation and since
>>> this may be scarce enabling user to disable this feature supports different use cases.
>>>
>>
>> Sure.
>>
>>>>
>>>> On AMD systems, io_alloc feature is backed by SDCIAE (L3 Smart Data Cache
>>>> Injection Allocation Enforcement). When enabled, SDCIAE directs all SDCI
>>>> lines to be placed into the L3 cache partitions specified by the register
>>>> corresponding to the highest CLOSID supported by the resource. With CDP
>>>> enabled, io_alloc routes I/O traffic using the highest CLOSID assigned to
>>>> the instruction cache (L3CODE).
>>>
>>> This is a lot of architecture specific text for a resctrl fs patch  ... I think
>>> you are trying to motivate the resctrl fs implementation. Similar motivation
>>> as proposed for cover letter can be used here to help explain the implementation
>>> choices.
>>
>> Updated the whole changelog.
>>
>> fs/resctrl: Add user interface to enable/disable io_alloc feature
>>
>> "io_alloc" feature in resctrl enables direct insertion of data from I/O
>> devices into the cache.
>>
>> Introduce user interface to enable/disable io_alloc feature.
> 
> The solution should be at end of changelog after description of problem it
> solves.

Sure. Moved this text below.

> 
>> On AMD systems, when io_alloc is enabled, the highest CLOSID is reserved
>> exclusively for I/O allocation traffic and is no longer available for
>> general CPU cache allocation. This feature is disabled by default. Users
> 
> Changelog should always be in imperative tone and problem and solution should
> be in separate paragraphs (above paragraph mixes problem and solution).  
> 
> For example, "Disable "io_alloc" feature by default to ensure all resources are
> available for general CPU cache allocation. ..." Although I do not think this is
> accurate since this patch does not do this?

Yes. Removed the text "This feature is disabled by default."
> 
>> are encouraged to enable it only when running workloads that can benefit
>> from this functionality.
>>
>> Since CLOSIDs are managed by resctrl fs, it is least invasive to make the
>> "io_alloc is supported by maximum supported CLOSID" part of the initial
>> resctrl fs support for io_alloc. Take care not to 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.
>>
> 
> The changelog requirements I refer to are documented in "Changelog" section
> of Documentation/process/maintainer-tip.rst. I feel like this should be
> familiar to you by now.

Sure,  Thank you.

-- 
Thanks
Babu Moger