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

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Date: Mon, 11 Mar 2024 10:40:35 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.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, yanjiewtw@...il.com,
 kim.phillips@....com, lukas.bulwahn@...il.com, seanjc@...gle.com,
 jmattson@...gle.com, leitao@...ian.org, jpoimboe@...nel.org,
 rick.p.edgecombe@...el.com, kirill.shutemov@...ux.intel.com,
 jithu.joseph@...el.com, kai.huang@...el.com, kan.liang@...ux.intel.com,
 daniel.sneddon@...ux.intel.com, pbonzini@...hat.com, sandipan.das@....com,
 ilpo.jarvinen@...ux.intel.com, peternewman@...gle.com,
 maciej.wieczor-retman@...el.com, 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 Reinette,

On 2/27/24 17:50, Reinette Chatre wrote:
> Hi Babu,
> 
> 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.

Thanks
Babu