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: <Z7cvP6YhcavIm0Av@e133380.arm.com>
Date: Thu, 20 Feb 2025 13:33:51 +0000
From: Dave Martin <Dave.Martin@....com>
To: "Moger, Babu" <babu.moger@....com>
Cc: Reinette Chatre <reinette.chatre@...el.com>,
	"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,
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	mario.limonciello@....com, james.morse@....com,
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	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

On Wed, Feb 19, 2025 at 12:07:30PM -0600, Moger, Babu wrote:
> 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.

Right, that's what I thought.

I'll defer to Reinette on whether it's important to keep the statement
about rationale -- it might indeed be easier to drop it if it just
raises more questions.

Cheers
---Dave