linux-kernel - Re: [PATCH v9 6/9] x86/resctrl: Add "mba_MBps_event" file to ctrl

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Message-ID: <Zz9w9FYAqbgu2aOq@agluck-desk3>
Date: Thu, 21 Nov 2024 09:42:12 -0800
From: "Luck, Tony" <tony.luck@...el.com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: Fenghua Yu <fenghua.yu@...el.com>,
	Peter Newman <peternewman@...gle.com>,
	Jonathan Corbet <corbet@....net>, x86@...nel.org,
	James Morse <james.morse@....com>,
	Jamie Iles <quic_jiles@...cinc.com>,
	Babu Moger <babu.moger@....com>,
	Randy Dunlap <rdunlap@...radead.org>,
	"Shaopeng Tan (Fujitsu)" <tan.shaopeng@...itsu.com>,
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	patches@...ts.linux.dev
Subject: Re: [PATCH v9 6/9] x86/resctrl: Add "mba_MBps_event" file to
 ctrl_mon directories

On Tue, Nov 19, 2024 at 08:03:06PM -0800, Reinette Chatre wrote:
> Hi Tony,
> 
> On 11/13/24 4:17 PM, Tony Luck wrote:
> > The "mba_MBps" mount option provides an alternate method to
> > control memory bandwidth. Instead of specifying allowable
> > bandwidth as a percentage of maximum possible, the user
> > provides a MiB/s limit value.
> > 
> > Historically the limit was enforced by a feedback loop from
> 
> "Historically the limit was enforced" no history needed since
> this is still the case at the time of this patch.
> 
> > the measure local bandwidth to adjust the memory bandwidth
> > allocation controls.
> 
> I am not sure what is meant by "a feedback loop from the measure
> local bandwidth" (that was copy&pasted to next patch).

Dropped the whole of the "Historically" paragraph.
> 
> > 
> > In preparation to allow the user to pick the memory bandwidth
> > monitoring event used as input to the feedback loop, provide
> > a file in each ctrl_mon group directory that shows the event
> 
> In the documentation the custom is to use CTRL_MON.

Fixed here (and in several other places in the series).

> > currently in use.
> 
> Much better changelog. I think it will help to add a snippet that
> mentions the file is only visible to user space if resctrl
> was mounted with mba_MBps, and thus only visible when mba_sc is
> enabled, and thus reinforcing that this maintains the contract
> that rdtgrp->mba_mbps_event is only accessed when mba_sc is enabled.

Added text to commit message about this file only being visible
when the "mba_MBps" mount option is in use.

> > 
> > Suggested-by: Reinette Chatre <reinette.chatre@...el.com>
> > Signed-off-by: Tony Luck <tony.luck@...el.com>
> > ---
> 
> Reinette

-Tony