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

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Message-ID: <aN6c3U8te/MK/WlW@e133380.arm.com>
Date: Thu, 2 Oct 2025 16:40:13 +0100
From: Dave Martin <Dave.Martin@....com>
To: "Chen, Yu C" <yu.c.chen@...el.com>
Cc: "Luck, Tony" <tony.luck@...el.com>, linux-kernel@...r.kernel.org,
	Reinette Chatre <reinette.chatre@...el.com>,
	James Morse <james.morse@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet <corbet@....net>,
	x86@...nel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH] fs/resctrl,x86/resctrl: Factor mba rounding to be
 per-arch

Hi there,

On Wed, Oct 01, 2025 at 08:13:45PM +0800, Chen, Yu C wrote:
> On 9/30/2025 11:55 PM, Dave Martin wrote:
> > Hi,
> > 
> > On Tue, Sep 30, 2025 at 12:43:36PM +0800, Chen, Yu C wrote:

[...]

> > > I overlooked this bit width. It should not exceed 511 according to the
> > > RDT spec. Previously, I was just wondering how to calculate the legacy
> > > MB percentage in Tony's example. If we want to keep consistency - if
> > > the user provides a value of 10, what is the denominator: Is it 255,
> > > 511, or something queried from ACPI.
> > > 
> > > MB: 0=4;1=75           <--- 10/255
> > > #MB_HW: 0=10;1=191
> > > #MB_MIN: 0=10;1=191
> > > #MB_MAX: 0=64;1=191
> > > 
> > > or
> > > 
> > > MB: 0=1;1=75          <--- 10/511
> > > #MB_HW: 0=10;1=191
> > > #MB_MIN: 0=10;1=191
> > > #MB_MAX: 0=64;1=191
> > > 
> > > thanks,
> > > Chenyu
> > 
> > The denomiator (the "scale" parameter in my model, though the name is
> > unimportant) should be whatever quantity of resource is specified in
> > the "unit" parameter.
> > 
> > For "percentage" type controls, I'd expect the unit to be 100% ("100pc"
> > in my syntax).
> > 
> > So, Tony suggestion looks plausible to me [1] :
> > 
> >   | Yes. 255 (or whatever "Q" value is provided in the ACPI table)
> >   | corresponds to no throttling, so 100% bandwidth.
> > 
> > So, if ACPI says Q=387, that's the denominator we advertise.
> > 
> > Does that sound right?
> > 
> 
> Yes, it makes sense, the denominator is the "scale" in your example.

Thanks for confirming that.

> > Question: is this a global parameter, or per-CPU?
> > 
> 
> It should be a global setting for all the MBA Register Blocks.

That's good -- since resctrl resource controls are not per-CPU,
exposing the exact hardware resolution won't work unless the value
is scaled identically for all CPUs.

Cheers
---Dave