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: <Z7Xc7Oxm9ByWhIEy@e133380.arm.com>
Date: Wed, 19 Feb 2025 13:30:20 +0000
From: Dave Martin <Dave.Martin@....com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: "Moger, Babu" <bmoger@....com>, Babu Moger <babu.moger@....com>,
	corbet@....net, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, tony.luck@...el.com,
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	paulmck@...nel.org, akpm@...ux-foundation.org, thuth@...hat.com,
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	pawan.kumar.gupta@...ux.intel.com, daniel.sneddon@...ux.intel.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

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?

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

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