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 10:15:32 -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>,
	<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 Babu,

On 3/12/2024 10:07 AM, Moger, Babu wrote:
> On 3/12/24 10:13, Reinette Chatre wrote:
>> 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.

I used the "original/legacy" text to make it clear which behavior I was
referring to. It was not a proposal for a label used by user space to 
select the behavior.

Isn't /sys/fs/resctrl/info/L3_MON/mbm_assign_control the file that will
assign the counters to domains? That should not be the file used to 
select the behavior. You had /sys/fs/resctrl/info/L3_MON/mbm_assign
with which user space selects behavior, no?


Reinette