linux-kernel - Re: [PATCH v6 19/22] x86/resctrl: Introduce the interface to switch between monitor modes

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Message-ID: <8c93c091-39b3-4219-b892-9df2747eb91a@intel.com>
Date: Fri, 16 Aug 2024 10:01:27 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: James Morse <james.morse@....com>, Babu Moger <babu.moger@....com>
CC: <x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>,
	<rdunlap@...radead.org>, <tj@...nel.org>, <peterz@...radead.org>,
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Subject: Re: [PATCH v6 19/22] x86/resctrl: Introduce the interface to switch
 between monitor modes

Hi James,

On 8/16/24 9:31 AM, James Morse wrote:
> Hi Babu,
> 
> On 06/08/2024 23:00, Babu Moger wrote:
>> Introduce interface to switch between ABMC and legacy modes.
>>
>> By default ABMC is enabled on boot if the feature is available.
>> Provide the interface to go back to legacy mode if required.
> 
> I may have missed it on an earlier version ... why would anyone want the non-ABMC
> behaviour on hardware that requires it: counters randomly reset and randomly return
> 'Unavailable'... is that actually useful?
> 
> You default this to on, so there isn't a backward compatibility argument here.
> 
> It seems like being able to disable this is a source of complexity - is it needed?

The ability to go back to legacy was added while looking ahead to support the next
"assignable counter" feature that is software based ("soft-RMID" .. "soft-ABMC"?).

This series adds support for ABMC on recent AMD hardware to address the issue described
in cover letter. This issue also exists on earlier AMD hardware that does not have the ABMC
feature and Peter is working on a software solution to address the issue on non-ABMC hardware.
This software solution is expected to have the same interface as the hardware solution but
earlier discussions revealed that it may introduce extra latency that users may only want to
accept during periods of active monitoring. Thus the option to disable the counter assignment
mode.

Your point about users returning to "legacy" mode on ABMC hardware is valid. I do not know
if that is useful. Here I can only speculate since monitoring with ABMC is
more accurate but also requires more user space involvement to assign counters while legacy
mode is less accurate while requiring less user space involvement.

> For MPAM I'm looking at enabling this on any platform that is short of monitors. If
> user-space disables it I don't have a "at random" hardware behaviour to fall back on - its
> extra work to invent a behaviour I'm not sure is useful...

It should not be required for MPAM to have a "legacy" mode. resctrl fs can expose only one
mode that is always enabled. Noting this now is important so that we can get the wording right
in the documentation.

Thanks for chiming in on MPAM's plans for this work.

Reinette