linux-kernel - Re: [PATCH v2 00/17] x86/resctrl : Support AMD Assignable Bandwidth Monitoring Counters (ABMC)

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Date: Tue, 12 Mar 2024 08:13:40 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....com>, James Morse <james.morse@....com>, <corbet@....net>,
	<fenghua.yu@...el.com>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<bp@...en8.de>, <dave.hansen@...ux.intel.com>
CC: <x86@...nel.org>, <hpa@...or.com>, <paulmck@...nel.org>,
	<rdunlap@...radead.org>, <tj@...nel.org>, <peterz@...radead.org>,
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	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<eranian@...gle.com>
Subject: Re: [PATCH v2 00/17] x86/resctrl : Support AMD Assignable Bandwidth
 Monitoring Counters (ABMC)

Hi Babu,

On 3/11/2024 8:40 AM, Moger, Babu wrote:
> On 2/27/24 17:50, Reinette Chatre wrote:
>> On 2/27/2024 10:12 AM, Moger, Babu wrote:
>>> On 2/26/24 15:20, Reinette Chatre wrote:
>>>> On 2/26/2024 9:59 AM, Moger, Babu wrote:
>>>>> On 2/23/24 16:21, Reinette Chatre wrote:
>>
>>>>>> Apart from the "default behavior" there are two options to consider ...
>>>>>> (a) the "original" behavior(? I do not know what to call it) - this would be
>>>>>>     where user space wants(?) to have the current non-ABMC behavior on an ABMC
>>>>>>     system, where the previous "num_rmids" monitor groups can be created but
>>>>>>     the counters are reset unpredictably ... should this still be supported
>>>>>>     on ABMC systems though?
>>>>>
>>>>> I would say yes. For some reason user(hardware or software issues) is not
>>>>> able to use ABMC mode, they have an option to go back to legacy mode.
>>>>
>>>> I see. Should this perhaps be protected behind the resctrl "debug" mount option?
>>>
>>> The debug option gives wrong impression. It is better to keep the option
>>> open to enable the feature in normal mode.
>>
>> You mentioned that it would only be needed when there are hardware or
>> software issues ... so debug does sound appropriate. Could you please give
>> an example of how debug option gives wrong impression? Why would you want
>> users to keep using "legacy" mode on an ABMC system?
> 
> We may not be able to use "-o debug" option to enable "legacy_mbm".
> With debug option it will always go to legcay mbm even if ABMC is supported.
> 
> For example when ABMC is supported, I cannot mount the resctrl with debug
> option to test ABMC.
> 
> I need to have a way to mount resctrl with ABMC and debug option. I can
> add "-o legacy_mbm" to enable lecacy_mbm.

I do not think it is necessary to add a unique debug option for this.
What if instead the "-o debug" mount option exposes the "original/legacy"
behavior in the control file? This would enable users to only use this
behavior when in "debug" mode while still able to switch between
legacy and assigned counters.

Reinette