linux-kernel - Re: [PATCH v12 18/26] x86/resctrl: Add default MBM event configurations for mbm_cntr

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Message-ID: <d6079947-2538-4258-8b1d-1f7b18a628f6@intel.com>
Date: Wed, 16 Apr 2025 09:21:02 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: "Luck, Tony" <tony.luck@...el.com>, "babu.moger@....com"
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Subject: Re: [PATCH v12 18/26] x86/resctrl: Add default MBM event
 configurations for mbm_cntr_assign mode

Hi Tony,

On 4/15/25 12:25 PM, Luck, Tony wrote:
>> By default, each resctrl group supports two MBM events: mbm_total_bytes
>> and mbm_local_bytes. These represent total and local memory bandwidth
>> monitoring, respectively. Each event corresponds to a specific MBM
>> configuration. Use these default configurations to set up the counters
>> during mount. Allow users to modify the configurations as needed after
>> initialization.
> 
> I think an update to this part of the resctrl.rst documentation is somewhat
> overdue:
> 
>         In a MON group these files provide a read out of the current
>         value of the event for all tasks in the group. In CTRL_MON groups
>         these files provide the sum for all tasks in the CTRL_MON group
>         and all tasks in MON groups. Please see example section for more
>         details on usage.
> 
> The sentence about CTRL_MON groups providing the sum for all tasks
> in the child MON groups is only true if counters are assigned to all of
> those MON groups. What mon_event_count() actually does is to
> return success if any of the CTRL_MON or child MON groups succeeded
> with the count being the sum of all the successes.

Thanks for catching this. This would be important to highlight so that
user space does not have impression that events of CTRL_MON can be
used as estimate for MON groups that do not have counters assigned.

Reinette