linux-kernel - Re: [PATCH v7 19/24] x86/resctrl: Report "Unassigned" for MBM events in mbm_cntr

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Message-ID: <c99c6b36-2618-4ca8-c716-e553637ddc11@amd.com>
Date: Fri, 27 Sep 2024 08:40:45 -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 v7 19/24] x86/resctrl: Report "Unassigned" for MBM events
 in mbm_cntr_assign mode

Hi Reinette,

On 9/26/2024 8:50 PM, Reinette Chatre wrote:
> Hi Babu,
> 
> On 9/26/24 12:16 PM, Moger, Babu wrote:
>> On 9/19/24 12:31, Reinette Chatre wrote:
>>> Hi Babu,
>>>
>>> On 9/4/24 3:21 PM, Babu Moger wrote:
>>>> In mbm_cntr_assign mode, the hardware counter should be assigned to read
>>>> the MBM events.
>>>>
>>>> Report "Unassigned" in case the user attempts to read the events without
>>>> assigning the counter.
>>>>
>>>> Signed-off-by: Babu Moger <babu.moger@....com>
>>>> ---
>>>> v7: Moved the documentation under "mon_data".
>>>>      Updated the text little bit.
>>>>
>>>> v6: Added more explaination in the resctrl.rst
>>>>      Added checks to detect "Unassigned" before reading RMID.
>>>>
>>>> v5: New patch.
>>>> ---
>>>>   Documentation/arch/x86/resctrl.rst        | 10 ++++++++++
>>>>   arch/x86/kernel/cpu/resctrl/ctrlmondata.c | 13 ++++++++++++-
>>>>   2 files changed, 22 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
>>>> index 3e9302971faf..ff5397d19704 100644
>>>> --- a/Documentation/arch/x86/resctrl.rst
>>>> +++ b/Documentation/arch/x86/resctrl.rst
>>>> @@ -417,6 +417,16 @@ When monitoring is enabled all MON groups will also contain:
>>>>   	for the L3 cache they occupy). These are named "mon_sub_L3_YY"
>>>>   	where "YY" is the node number.
>>>>   
>>>> +	The mbm_cntr_assign mode allows users to assign a hardware counter
>>>> +	to an RMID-event pair, enabling bandwidth monitoring for as long
>>>> +	as the counter remains assigned. The hardware will continue tracking
>>>> +	the assigned RMID until the user manually unassigns it, ensuring
>>>> +	that counters are not reset during this period. With a limited number
>>>> +	of counters, the system may run out of assignable resources. In
>>>> +	mbm_cntr_assign mode, MBM event counters will return "Unassigned"
>>>> +	if the counter is not allocated to the event when read. Users must
>>>> +	manually assign a counter to read the events.
>>>> +
>>>
>>> Please consider how this text could also be relevant to soft-ABMC.
>>
>> It mostly applies to soft-ABMC also. Minor tweaking may be required.
> 
> hmmm ... seems that I still have mostly the "soft-RMID" model in my head.
> 
>> How about?
>>
>> "When supported the 'mbm_cntr_assign' mode allows users to assign a
>> hardware counter to RMID, event pair, enabling bandwidth monitoring for as
> 
> hmmm ... so soft-ABMC also assigns hardware counters?


It does not have hardware counter. I need to change this text.

> Also, we should aim for generic text that will cover how this may look on MPAM
> also. Considering this, it may just mean to replace "RMID, event pair" with
> "mon_hw_id, event pair"?

ok.

> 
>> long as the counter remains assigned. The hardware will continue tracking
>> the assigned RMID until the user manually unassigns it, ensuring
> 
> Please do double-check all usage of "RMID" in user facing interfaces/docs where
> mon_hw_id may be more appropriate.

Sure.

> 
>> that counters are not reset during this period. With a limited number
>> of counters, the system may run out of assignable counters at some point.
>> In that case, MBM event counters will return "Unassigned" when the event
>> when read. Users must manually assign a counter to read the events."
> 
> "when the event when read" -> "when the event is read"?

Sure.

-- 
- Babu Moger