linux-kernel - Re: [PATCH v11 11/23] x86/resctrl: Introduce mbm_cntr_cfg to track assignable counters at domain

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Message-ID: <21c911af-d396-411a-abdf-f72dd96d92c0@amd.com>
Date: Wed, 19 Feb 2025 12:07:30 -0600
From: "Moger, Babu" <babu.moger@....com>
To: Dave Martin <Dave.Martin@....com>,
 Reinette Chatre <reinette.chatre@...el.com>
Cc: "Moger, Babu" <bmoger@....com>, corbet@....net, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
 tony.luck@...el.com, peternewman@...gle.com, x86@...nel.org, hpa@...or.com,
 paulmck@...nel.org, akpm@...ux-foundation.org, thuth@...hat.com,
 rostedt@...dmis.org, xiongwei.song@...driver.com,
 pawan.kumar.gupta@...ux.intel.com, daniel.sneddon@...ux.intel.com,
 jpoimboe@...nel.org, perry.yuan@....com, sandipan.das@....com,
 kai.huang@...el.com, xiaoyao.li@...el.com, seanjc@...gle.com,
 xin3.li@...el.com, andrew.cooper3@...rix.com, ebiggers@...gle.com,
 mario.limonciello@....com, james.morse@....com, tan.shaopeng@...itsu.com,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 maciej.wieczor-retman@...el.com, eranian@...gle.com
Subject: Re: [PATCH v11 11/23] x86/resctrl: Introduce mbm_cntr_cfg to track
 assignable counters at domain

Hi Dave,

On 2/19/25 07:30, Dave Martin wrote:
> Hi,
> 
> On Mon, Feb 10, 2025 at 10:10:26AM -0800, Reinette Chatre wrote:
>> Hi Babu,
>>
>> On 2/7/25 10:23 AM, Moger, Babu wrote:
>>> On 2/5/2025 5:57 PM, Reinette Chatre wrote:
>>>> On 1/22/25 12:20 PM, Babu Moger wrote:
> 
> [...]
> 
>>>>> MBM events of a monitoring group is tracked by hardware. Such queries
>>>>> are acceptable because of a very small number of assignable counters.
>>>>
>>>> It is not obvious what "very small number" means. Is it possible to give
>>>> a range to help reader understand the motivation?
>>>
>>> How about?
>>>
>>> MBM events of a monitoring group is tracked by hardware. Such queries
>>> are acceptable because of a very small number of assignable counters(32 to 64).
>>
>> Yes, thank you. This helps to understand the claim.
>>
>> Reinette
> 
> Do these queries only happen when userspace reads an mbm_assign_control
> file?

Yes. All these queries are initiated by userspace in the form of
individual assignments or creating a group(mkdir).

> 
> It might be worth documenting somewhere that writing and (especially)
> reading an mbm_assign_control file is not intended to be super-fast.


We can drop the last sentence if it is creating confusion.

> 
> It feels like userspace should not generally rely on reading
> mbm_assign_control files except for diagnostic purposes, or occasional
> read-modify-write transformations.  Or do expect some other usage model
> that makes this a hotter path?
> 
> Cheers
> ---Dave

Our earlier interface was intended to query each group separately. After
the input from Peter, we changed it to batched query. One query from
userspace can list all the assignments. I am not aware of any other usage
model.
-- 
Thanks
Babu Moger