linux-kernel - Re: [PATCH v4 00/19] x86/resctrl : Support AMD Assignable Bandwidth Monitoring Counters (ABMC)

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Message-ID: <d98087ff-8038-3f57-fc3b-df745d55141a@amd.com>
Date: Fri, 21 Jun 2024 11:41:07 -0500
From: "Moger, Babu" <bmoger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, 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: [PATCH v4 00/19] x86/resctrl : Support AMD Assignable Bandwidth
 Monitoring Counters (ABMC)

Hi Reinette,

On 6/20/2024 5:49 PM, Reinette Chatre wrote:
> Hi Babu,
> 
> On 6/18/24 2:02 PM, Moger, Babu wrote:
>> On 6/13/24 19:54, Reinette Chatre wrote:
>>> On 5/24/24 5:23 AM, Babu Moger wrote:
>>>
>>>>     when reading, then the read will come back as Unavailable.
>>>
>>> Should this not rather be "Unassigned"? According to the docs the 
>>> counters
>>> will return "Unavailable" right after reconfigure so it seems that there
>>> are scenarios where an "assigned" counter returns "Unavailable". It 
>>> seems
>>> more
>>> useful to return "Unassigned" that will have a new specific meaning that
>>> overloading existing "Unavailable" that has original meaning of "try
>>> again" ....
>>> but in this case trying again will be futile.
>>
>> Hardware returns "Unavailable" in both the cases. So, thought of 
>> reporting the same without any interpretation.
>>
> 
> I do not see these as the same. When a counter is assigned and its
> read returns "Unavailable" then the user reasonably expects that
> retry will work.
> When a counter is not assigned then no amount of retries will result
> in data. How is user space expected to distinguish between the two
> scenarios that return the same error with such significantly different
> meaning? rdtgroup_mondata_shows() can just peek into the monitor
> group state and see if a counter is assigned and immediately
> return "Unassigned" if no counter is assigned, no? I see no need
> for it to IPI another CPU and try to read a counter that it already knows
> will be futile. This seems unnecessary and the generic "Unavailable" is
> not helpful to user space.
> 

Sure. Will change it to report "Unassigned" in these cases.
Thanks
-- 
- Babu Moger