linux-kernel - Re: [PATCH v15 33/34] fs/resctrl: Introduce the interface to switch between monitor modes

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Message-ID: <af0a7a35-2f6d-4c8c-990d-af809d9547bf@intel.com>
Date: Thu, 17 Jul 2025 21:03:02 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Babu Moger <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>,
	<kai.huang@...el.com>, <xiaoyao.li@...el.com>, <peterz@...radead.org>,
	<xin3.li@...el.com>, <kan.liang@...ux.intel.com>,
	<mario.limonciello@....com>, <thomas.lendacky@....com>, <perry.yuan@....com>,
	<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 33/34] fs/resctrl: Introduce the interface to switch
 between monitor modes

Hi Babu,

On 7/8/25 3:17 PM, Babu Moger wrote:
> Resctrl subsystem can support two monitoring modes, "mbm_event" or
> "default". In mbm_event mode, monitoring event can only accumulate data
> while it is backed by a hardware counter. In "default" mode, resctrl
> assumes there is a hardware counter for each event within every CTRL_MON
> and MON group.
> 
> Introduce mbm_assign_mode resctrl file to switch between mbm_event and
> default modes.
> 
> Example:
> To list the MBM monitor modes supported:
> $ cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> [mbm_event]
> default
> 
> To enable the "mbm_event" monitoring mode:

"monitoring mode" -> "counter assignment mode"?

> $ echo "mbm_event" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> 
> To enable the "default" monitoring mode:
> $ echo "default" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> 
> MBM event counters are automatically reset as part of changing the mode.
> Clear both architectural and non-architectural event states to prevent
> overflow conditions during the next event read. Also clear assignable
> counter configuration on all the domains.
> 
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---

...

> ---
>  Documentation/filesystems/resctrl.rst | 22 +++++++-
>  fs/resctrl/internal.h                 |  2 +
>  fs/resctrl/monitor.c                  | 27 ++++++++++
>  fs/resctrl/rdtgroup.c                 | 72 ++++++++++++++++++++++++++-
>  4 files changed, 121 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
> index 2b2acb55d8b1..b8a4dbe061ea 100644
> --- a/Documentation/filesystems/resctrl.rst
> +++ b/Documentation/filesystems/resctrl.rst
> @@ -259,7 +259,8 @@ with the following files:
>  
>  "mbm_assign_mode":
>  	The supported counter assignment modes. The enclosed brackets indicate which mode
> -	is enabled.
> +	is enabled. The MBM events associated with counters may reset when "mbm_assign_mode"
> +	is changed.
>  	::
>  
>  	  # cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> @@ -279,6 +280,15 @@ with the following files:
>  	of counters available is described in the "num_mbm_cntrs" file. Changing the
>  	mode may cause all counters on the resource to reset.
>  
> +	Moving to mbm_event counter assignment mode requires users to assign the counters
> +	to the events. Otherwise, the MBM event counters will return 'Unassigned' when read.
> +
> +	The mode is beneficial for AMD platforms that support more CTRL_MON
> +	and MON groups than available hardware counters. By default, this
> +	feature is enabled on AMD platforms with the ABMC (Assignable Bandwidth
> +	Monitoring Counters) capability, ensuring counters remain assigned even
> +	when the corresponding RMID is not actively used by any processor.
> +
>  	"default":
>  
>  	In default mode, resctrl assumes there is a hardware counter for each
> @@ -288,6 +298,16 @@ with the following files:
>  	result in misleading values or display "Unavailable" if no counter is assigned
>  	to the event.
>  
> +	* To enable "mbm_event" monitoring mode:

"monitoring mode" -> "counter assignment mode"?

> +	  ::
> +
> +	    # echo "mbm_event" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> +
> +	* To enable "default" monitoring mode:
> +	  ::
> +
> +	    # echo "default" > /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> +
>  "num_mbm_cntrs":
>  	The maximum number of counters (total of available and assigned counters) in
>  	each domain when the system supports mbm_event mode.

Reinette