linux-kernel - Re: [PATCH v15 24/34] fs/resctrl: Report 'Unassigned' for MBM events in mbm

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Message-ID: <791ff5e0-dcd8-4bfc-810c-b085bc4ca543@intel.com>
Date: Tue, 22 Jul 2025 16:28:17 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....com>, <corbet@....net>, <tony.luck@...el.com>,
	<james.morse@....com>, <tglx@...utronix.de>, <mingo@...hat.com>,
	<bp@...en8.de>, <dave.hansen@...ux.intel.com>
CC: <Dave.Martin@....com>, <x86@...nel.org>, <hpa@...or.com>,
	<akpm@...ux-foundation.org>, <paulmck@...nel.org>, <rostedt@...dmis.org>,
	<Neeraj.Upadhyay@....com>, <david@...hat.com>, <arnd@...db.de>,
	<fvdl@...gle.com>, <seanjc@...gle.com>, <jpoimboe@...nel.org>,
	<pawan.kumar.gupta@...ux.intel.com>, <xin@...or.com>,
	<manali.shukla@....com>, <tao1.su@...ux.intel.com>, <sohil.mehta@...el.com>,
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	<gautham.shenoy@....com>, <chang.seok.bae@...el.com>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<peternewman@...gle.com>, <eranian@...gle.com>
Subject: Re: [PATCH v15 24/34] fs/resctrl: Report 'Unassigned' for MBM events
 in mbm_event mode

Hi Babu,

On 7/22/25 11:15 AM, Moger, Babu wrote:
> Hi Reinette,
> 
> On 7/17/25 22:53, Reinette Chatre wrote:
>> Hi Babu,
>>
>> On 7/8/25 3:17 PM, Babu Moger wrote:
>>> When the "mbm_event" counter assignment mode is enabled, a hardware counter
>>> must be assigned to read the event.
>>>
>>> Report 'Unassigned' in case the user attempts to read the event without
>>> assigning a hardware counter.
>>>
>>> Signed-off-by: Babu Moger <babu.moger@....com>
>>> ---
>>
>>
>>
>>> ---
>>>  Documentation/filesystems/resctrl.rst | 8 ++++++++
>>>  fs/resctrl/ctrlmondata.c              | 6 ++++++
>>>  2 files changed, 14 insertions(+)
>>>
>>> diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
>>> index 446736dbd97f..4f5eb5bbd4b5 100644
>>> --- a/Documentation/filesystems/resctrl.rst
>>> +++ b/Documentation/filesystems/resctrl.rst
>>> @@ -434,6 +434,14 @@ 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_event" counter assignment mode offers "num_mbm_cntrs" number of
>>> +	counters and allows users to assign counters to mon_hw_id, event pairs
>>> +	enabling bandwidth monitoring for as long as the counter remains assigned.
>>> +	The hardware will continue tracking the assigned counter until the user
>>> +	manually unassigns it, ensuring that event data is not reset during this
>>> +	period. An MBM event returns 'Unassigned' when the event does not have
>>> +	a hardware counter assigned.
>>
>> Most of this duplicates the "mbm_event" description added in patch #10. It should just
>> be sufficient to mention that this applies to "mbm_event" counter assignment mode
>> and then user can look up the main description in the doc.
>>
>> The last sentence is related to this section and need an update to reflect behavior
>> when a CTRL_MON event is read and it or some of the MON groups do not have
>> counters assigned. The paragraph that precedes this does describe how the sum
>> works so this can tie into that.
> 
> Just added following text.
> 
> When the 'mbm_event' counter assignment mode is enabled, reading

Not sure how this will turn out ... if I understand correctly it follows a
paragraph that already starts with "The "mbm_event" counter assignment mode offers ..."
so there seems to be some redundancy.

> an MBM event returns 'Unassigned' if no hardware counter is assigned

How about "reading an MBM event" -> "reading an MBM of a MON group" to
distinguish it from the text about CTRL_MON group that follows it?

> to it. For CTRL_MON groups, 'Unassigned' is returned if none of the
> events in the CTRL_MON group or its associated MON groups have assigned
> counters.
> 

Reinette