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 12:07:27 -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 3/12/24 10:13, Reinette Chatre wrote:
> 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.

It makes the code simple.

> What if instead the "-o debug" mount option exposes the "original/legacy"

Can you please elaborate on this?

Did you mean following command to enable legacy mode?

$echo "original/legacy"    /sys/fs/resctrl/info/L3_MON/mbm_assign_control

It feels like a overkill and confusing.

> 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
> 

-- 
Thanks
Babu Moger