Message-ID: <8ce71c26-395a-4d04-ae75-5d54dcaf51d4@amd.com>
Date: Wed, 6 Aug 2025 16:02:59 -0500
From: "Moger, Babu" <babu.moger@....com>
To: corbet@....net, tony.luck@...el.com, reinette.chatre@...el.com,
 james.morse@....com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
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Subject: Re: [PATCH v16 10/34] fs/resctrl: Introduce the interface to display
 monitoring modes

Hi Reinette,

On 7/25/25 13:29, Babu Moger wrote:
> Introduce the resctrl file "mbm_assign_mode" to list the supported counter
> assignment modes.
> 
> The "mbm_event" counter assignment mode allows users to assign a hardware
> counter to an RMID, event pair and monitor bandwidth usage as long as it is
> assigned. The hardware continues to track the assigned counter until it is
> explicitly unassigned by the user. Each event within a resctrl group can be
> assigned independently in this mode.
> 
> On AMD systems "mbm_event" mode is backed by the ABMC (Assignable
> Bandwidth Monitoring Counters) hardware feature and is enabled by default.
> 
> The "default" mode is the existing mode that works without the explicit
> counter assignment, instead relying on dynamic counter assignment by
> hardware that may result in hardware not dedicating a counter resulting
> in monitoring data reads returning "Unavailable".
> 
> Provide an interface to display the monitor modes on the system.
> 
> $ cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> [mbm_event]
> default
> 
> Add IS_ENABLED(CONFIG_RESCTRL_ASSIGN_FIXED) check to support Arm64.
> 
> On x86, CONFIG_RESCTRL_ASSIGN_FIXED is not defined. On Arm64, it will be
> defined when the "mbm_event" mode is supported.
> 
> Add IS_ENABLED(CONFIG_RESCTRL_ASSIGN_FIXED) check early to ensure the user
> interface remains compatible with upcoming Arm64 support. IS_ENABLED()
> safely evaluates to 0 when the configuration is not defined.
> 
> As a result, for MPAM, the display would be either:
> [default]
> or
> [mbm_event]
> 
> Signed-off-by: Babu Moger <babu.moger@....com>
> Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>
> ---
> v16: Update with Reviewed-by tag.
> 
> v15: Minor text changes in changelog and resctrl.rst.
> 
> v14: Changed the name of the monitor mode to mbm_cntr_evt_assign based on the discussion.
>      https://lore.kernel.org/lkml/7628cec8-5914-4895-8289-027e7821777e@amd.com/
>      Changed the name of the mbm_assign_mode's.
>      Updated resctrl.rst for mbm_event mode.
>      Changed subject line to fs/resctrl.
> 
> v13: Updated the commit log with motivation for adding CONFIG_RESCTRL_ASSIGN_FIXED.
>      Added fflag RFTYPE_RES_CACHE for mbm_assign_mode file.
>      Updated user doc. Removed the references to "mbm_assign_control".
>      Resolved the conflicts with latest FS/ARCH code restructure.
> 
> v12: Minor text update in change log and user documentation.
>      Added the check CONFIG_RESCTRL_ASSIGN_FIXED to take care of arm platforms.
>      This will be defined only in arm and not in x86.
> 
> v11: Renamed rdtgroup_mbm_assign_mode_show() to resctrl_mbm_assign_mode_show().
>      Removed few texts in resctrl.rst about AMD specific information.
>      Updated few texts.
> 
> v10: Added few more text to user documentation clarify on the default mode.
> 
> v9: Updated user documentation based on comments.
> 
> v8: Commit message update.
> 
> v7: Updated the descriptions/commit log in resctrl.rst to generic text.
>     Thanks to James and Reinette.
>     Rename mbm_mode to mbm_assign_mode.
>     Introduced mutex lock in rdtgroup_mbm_mode_show().
> 
> v6: Added documentation for mbm_cntr_assign and legacy mode.
>     Moved mbm_mode fflags initialization to static initialization.
> 
> v5: Changed interface name to mbm_mode.
>     It will be always available even if ABMC feature is not supported.
>     Added description in resctrl.rst about ABMC mode.
>     Fixed display abmc and legacy consistantly.
> 
> v4: Fixed the checks for legacy and abmc mode. Default it ABMC.
> 
> v3: New patch to display ABMC capability.
> ---
>  Documentation/filesystems/resctrl.rst | 31 ++++++++++++++++++++++
>  fs/resctrl/rdtgroup.c                 | 37 +++++++++++++++++++++++++++
>  2 files changed, 68 insertions(+)
> 
> diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
> index c97fd77a107d..b692829fec5f 100644
> --- a/Documentation/filesystems/resctrl.rst
> +++ b/Documentation/filesystems/resctrl.rst
> @@ -257,6 +257,37 @@ with the following files:
>  	    # cat /sys/fs/resctrl/info/L3_MON/mbm_local_bytes_config
>  	    0=0x30;1=0x30;3=0x15;4=0x15
>  
> +"mbm_assign_mode":
> +	The supported counter assignment modes. The enclosed brackets indicate which mode
> +	is enabled.
> +	::
> +
> +	  # cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> +	  [mbm_event]
> +	  default
> +
> +	"mbm_event":
> +
> +	mbm_event mode allows users to assign a hardware counter to an RMID, event
> +	pair and monitor the bandwidth usage as long as it is assigned. The hardware
> +	continues to track the assigned counter until it is explicitly unassigned by
> +	the user. Each event within a resctrl group can be assigned independently.
> +
> +	In this mode, a monitoring event can only accumulate data while it is backed
> +	by a hardware counter. Use "mbm_L3_assignments" found in each CTRL_MON and MON
> +	group to specify which of the events should have a counter assigned. The number
> +	of counters available is described in the "num_mbm_cntrs" file. Changing the
> +	mode may cause all counters on the resource to reset.
> +
> +	"default":
> +
> +	In default mode, resctrl assumes there is a hardware counter for each
> +	event within every CTRL_MON and MON group. On AMD platforms, it is
> +	recommended to use the mbm_event mode, if supported, to prevent reset of MBM
> +	events between reads resulting from hardware re-allocating counters. This can
> +	result in misleading values or display "Unavailable" if no counter is assigned
> +	to the event.
> +
>  "max_threshold_occupancy":
>  		Read/write file provides the largest value (in
>  		bytes) at which a previously used LLC_occupancy
> diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
> index ca0475b75390..c7ca9113a12a 100644
> --- a/fs/resctrl/rdtgroup.c
> +++ b/fs/resctrl/rdtgroup.c
> @@ -1799,6 +1799,36 @@ static ssize_t mbm_local_bytes_config_write(struct kernfs_open_file *of,
>  	return ret ?: nbytes;
>  }
>  
> +static int resctrl_mbm_assign_mode_show(struct kernfs_open_file *of,
> +					struct seq_file *s, void *v)
> +{
> +	struct rdt_resource *r = rdt_kn_parent_priv(of->kn);
> +	bool enabled;
> +
> +	mutex_lock(&rdtgroup_mutex);
> +	enabled = resctrl_arch_mbm_cntr_assign_enabled(r);
> +
> +	if (r->mon.mbm_cntr_assignable) {
> +		if (enabled)
> +			seq_puts(s, "[mbm_event]\n");
> +		else
> +			seq_puts(s, "[default]\n");
> +
> +		if (!IS_ENABLED(CONFIG_RESCTRL_ASSIGN_FIXED)) {
> +			if (enabled)
> +				seq_puts(s, "default\n");
> +			else
> +				seq_puts(s, "mbm_event\n");
> +		}
> +	} else {
> +		seq_puts(s, "[default]\n");
> +	}
> +
> +	mutex_unlock(&rdtgroup_mutex);
> +
> +	return 0;
> +}

The resctrl_mbm_assign_mode_show() can also be moved to monitor.c.

What do you think?



-- 
Thanks
Babu Moger

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