linux-kernel - Re: [RFC PATCH v3 00/17] x86/resctrl : Support AMD Assignable Bandwidth Monitoring Counters (ABMC)

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Date: Thu, 9 May 2024 11:07:12 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....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>, <james.morse@....com>
Subject: Re: [RFC PATCH v3 00/17] x86/resctrl : Support AMD Assignable
 Bandwidth Monitoring Counters (ABMC)

Hi Babu,

On 5/8/2024 4:29 PM, Moger, Babu wrote:
> On 5/8/24 15:41, Reinette Chatre wrote:
>> On 5/8/2024 1:07 PM, Moger, Babu wrote:
>>> On 5/7/24 15:26, Reinette Chatre wrote:
>>>> On 5/6/2024 10:18 AM, Moger, Babu wrote:
>>>>> On 5/3/24 18:24, Reinette Chatre wrote:
>>>>>> On 3/28/2024 6:06 PM, Babu Moger wrote:
>>>>>>
>>>>>>> a. Check if ABMC support is available
>>>>>>> 	#mount -t resctrl resctrl /sys/fs/resctrl/
>>>>>>>
>>>>>>> 	#cat /sys/fs/resctrl/info/L3_MON/mbm_assign
>>>>>>> 	[abmc] 
>>>>>>> 	legacy_mbm
>>>>>>>
>>>>>>> 	Linux kernel detected ABMC feature and it is enabled.
>>>>>>
>>>>>> Please note that this adds the "abmc" feature to the resctrl
>>>>>> *filesystem* that supports more architectures than just AMD. Calling the
>>>>>> resctrl filesystem feature "abmc" means that (a) AMD needs to be ok with
>>>>>> other architectures calling their features that are
>>>>>> similar-but-maybe-not-identical-to-AMD-ABMC "abmc", or (b) this needs
>>>>>> a new generic name.
>>>>>
>>>>> It should not a problem if other architecture calling abmc for similar
>>>>> feature. But generic name is always better if there is a suggestion.
>>>>
>>>> "should not a problem" does not instill confidence that AMD is
>>>> actually ok with this.
>>>
>>> The feature "ABMC" has been used in the public document already to refer
>>> this feature.
>>> https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24594.pdf
>>
>> It is clear to me that Assignable Bandwidth Monitoring Counters (ABMC) is the
>> name of the AMD feature. The question is whether users can use the 
>> same name to interact with "similar but maybe not identical" features from other
>> architectures, which is what this series enables.
>>
>>> If there comes a conflict then we can change it to amd_abmc. Didn't see
>>> any conflict at this pint.
>>
>> How do you envision this? The resctrl filesystem interface is intended to be
>> architecture neutral so it is not obvious to me how "amd_abmc" is expected
>> to look? Why would it be necessary to have different architecture specific names
>> for a similar feature from different architectures that users interact with in
>> the same way? Sounds to me as though this just needs a new non-AMD marketing name. 
> 
> I think I misunderstood it.
> It is not a concern to have a same name("abmc") for similar feature across
> the architectures.

Thank you for confirming. This joins BMEC and SMBA in this regard.

> ABMC is also kind of generic. I am open to other generic suggestions. I
> think we should have "assign" and "monitor" words in them.

One thing we can consider is to move to a simple "enable"/"disable"
interface for events. Users do not really need to know that hardware
needs to "assign a counter" to an event for it to measure. Yes,
user space can infer some of this by the number of events that
can be "enabled" at a time, but the concept of "assign a hardware counter"
is abstract and does not directly map to (as I understand) the soft-RMID
approach for other AMD hardware. Peter reminded us a while back [1] that
"assign" has a variety of meanings, even among AMD, so we should aim to
avoid any confusion. 

Reinette

[1] https://lore.kernel.org/lkml/CALPaoCjg-W3w8OKLHP_g6Evoo03fbgaOQZrGTLX6vdSLp70=SA@mail.gmail.com/