linux-kernel - Re: [PATCH v12 16/26] x86/resctrl: Report 'Unassigned' for MBM events in mbm_cntr

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Message-ID: <4bf73e0d-3533-43af-9622-94d70425d22c@intel.com>
Date: Fri, 11 Apr 2025 14:08:54 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Babu Moger <babu.moger@....com>, <tony.luck@...el.com>,
	<peternewman@...gle.com>
CC: <corbet@....net>, <tglx@...utronix.de>, <mingo@...hat.com>,
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Subject: Re: [PATCH v12 16/26] x86/resctrl: Report 'Unassigned' for MBM events
 in mbm_cntr_assign mode

Hi Babu,

On 4/3/25 5:18 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

"the events" -> "the event"?

> assigning the counter.

"the counter" -> "a hardware counter"?

> 
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---
...

> ---
>  Documentation/arch/x86/resctrl.rst        | 10 ++++++++++
>  arch/x86/kernel/cpu/resctrl/ctrlmondata.c | 14 ++++++++++++++
>  arch/x86/kernel/cpu/resctrl/internal.h    |  3 +++
>  arch/x86/kernel/cpu/resctrl/monitor.c     |  4 ++--
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c    |  2 +-
>  5 files changed, 30 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index 44128fbda4fe..71ed1cfed33a 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -430,6 +430,16 @@ 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 offers "num_mbm_cntrs" number of counters
> +	and allows users to assign a counter to mon_hw_id, event pair enabling
> +	bandwidth monitoring for as long as the counter remains assigned.
> +	The hardware will continue tracking the assigned mon_hw_id until
> +	the user manually unassigns it, ensuring that counters are not reset
> +	during this period. System may run out of assignable counters when
> +	all the counters are already assigned. In that case, MBM event counters

Counters could be unassigned even if there are assignable counters available.

I think the "System may run ..." sentence should be dropped.
The "In that case ..." sentence could be simplified with something like:
"An MBM event returns 'Unassigned' when the event does not have a hardware
counter assigned."

> +	will return 'Unassigned' when the event is read. Users must manually
> +	assign a counter to read the events.
> +
>  "mon_hw_id":
>  	Available only with debug option. The identifier used by hardware
>  	for the monitor group. On x86 this is the RMID.

Reinette