Re: [PATCH] fs/resctrl,x86/resctrl: Factor mba rounding to be per-arch

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Dave,

On 9/29/25 5:43 AM, Dave Martin wrote:
> Hi Reinette,
> 
> On Thu, Sep 25, 2025 at 09:53:37PM +0100, Reinette Chatre wrote:
>> Hi Dave,
>>
>> On 9/25/25 5:46 AM, Dave Martin wrote:
>>> On Tue, Sep 23, 2025 at 10:27:40AM -0700, Reinette Chatre wrote:
>>>> On 9/22/25 7:39 AM, Dave Martin wrote:
>>>>> On Fri, Sep 12, 2025 at 03:19:04PM -0700, Reinette Chatre wrote:
>>>>>> Hi Dave,
> 
> [...]
> 
>>>>>> Also please use upper case for acronym mba->MBA.
>>>>>
>>>>> Ack (the local custom in the MPAM code is to use "mba", but arguably,
>>>>> the meaning is not quite the same -- I'll change it.)
>>>>
>>>> I am curious what the motivation is for the custom? Knowing this will help
>>>> me to keep things consistent when the two worlds meet.
>>>
>>> I think this has just evolved over time.  On the x86 side, MBA is a
>>> specific architectural feature, but on the MPAM side the architecture
>>> doesn't really have a name for the same thing.  Memory bandwidth is a
>>> concept, but a few different types of control are defined for it, with
>>> different names.
>>>
>>> So, for the MPAM driver "mba" is more of a software concept than
>>> something in a published spec: it's the glue that attaches to "MB"
>>> resource as seen through resctrl.
>>>
>>> (This isn't official though; it's just the mental model that I have
>>> formed.)
>>
>> I see. Thank you for the details. My mental model is simpler: write acronyms
>> in upper case.
> 
> Generally, I agree, although I'm not sure whether that acronym belongs
> in the MPAM-specific code.
> 
> For this patch, though, that's irrelevant.  I've changed it to "MBA"
> as requested.
> 

Thank you.

...

>>>> Considering the two statements:
>>>> - "The available steps are no larger than this value."
>>>> - "this value ... is not smaller than the apparent size of any individual rounding step"
>>>>
>>>> The "not larger" and "not smaller" sounds like all these words just end up saying that
>>>> this is the step size?
>>>
>>> They are intended to be the same statement: A <= B versus
>>> B >= A respectively.
>>
>> This is what I understood from the words ... and that made me think that it
>> can be simplified to A = B ... but no need to digress ... onto the alternatives below ...
> 
> Right...
> 
> [...]
> 
>>> Instead, maybe we can just say something like:
>>>
>>>  | The available steps are spaced at roughly equal intervals between the
>>>  | value reported by info/MB/min_bandwidth and 100%, inclusive.  Reading
>>>  | info/MB/bandwidth_gran gives the worst-case precision of these
>>>  | interval steps, in per cent.
>>>
>>> What do you think?
>>
>> I find "worst-case precision" a bit confusing, consider for example, what
>> would "best-case precision" be? What do you think of "info/MB/bandwidth_gran gives
>> the upper limit of these interval steps"? I believe this matches what you
>> mentioned a couple of messages ago: "The available steps are no larger than this
>> value."
> 
> Yes, that works.  "Worst case" implies a value judgement that smaller
> steps are "better" then large steps, since the goal is control.
> 
> But your wording, to the effect that this is the largest (apparent)
> step size, conveys all the needed information.

Thank you for considering it. My preference is for stating things succinctly
and not leave too much for interpretation.

> 
>> (and "per cent" -> "percent")
> 
> ( Note: https://en.wiktionary.org/wiki/per_cent )

Yes, in particular I note the "chiefly Commonwealth". I respect the differences
in the English language and was easily convinced in earlier MPAM work to
accept different spelling. I now regret doing so because after merge we now have a
supposedly coherent resctrl codebase with inconsistent spelling that is unpleasant
to encounter when reading the code and also complicates new work.

> (Though either is acceptable, the fused word has a more informal feel
> to it for me.  Happy to change it -- though your rewording below gets
> rid of it anyway.  (This word doesn't appear in resctrl.rst --
> evertying is "percentage" etc.)
> 
>>
>>> If that's adequate, then the wording under the definition of
>>> "bandwidth_gran" could be aligned with this.
>>
>> I think putting together a couple of your proposals and statements while making the
>> text more accurate may work:
>>
>> 	 "bandwidth_gran":
>> 		The approximate granularity in which the memory bandwidth
>>  		percentage is allocated. The allocated bandwidth percentage
>> 		is rounded up to the next control step available on the
>> 		hardware. The available hardware steps are no larger than
>> 		this value.
> 
> That's better, thanks.  I'm happy to pick this up and reword the text
> in both places along these lines.

Thank you very much. Please do feel free to rework.

> 
>> I assume "available" is needed because, even though the steps are not larger
>> than "bandwidth_gran", the steps may not be consistent across the "min_bandwidth"
>> to 100% range?
> 
> Yes -- or, rather, the steps _look_ inconsistent because they are
> rounded to exact percentages by the interface.
> 
> I don't think we expect the actual steps in the hardware to be
> irregular.
> 
Thank you for clarifying.

Reinette