Re: [PATCH v12 08/26] x86/resctrl: Introduce the interface to display monitor mode

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Babu,

On 4/3/25 5:18 PM, Babu Moger wrote:
> Introduce the interface file "mbm_assign_mode" to list monitor modes

Using "resctrl file" instead of "interface file" should help to make it
clear what this patch does.

> supported.
> 
> The "mbm_cntr_assign" mode provides the option to assign a counter to
> an RMID, event pair and monitor the bandwidth as long as it is assigned.
> 
> On AMD systems "mbm_cntr_assign" mode is backed by the ABMC (Assignable
> Bandwidth Monitoring Counters) hardware feature and is enabled by default.
> 
> The "default" mode is the existing monitoring 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 mode on the system.
> $ cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> [mbm_cntr_assign]
> default
> 
> Added an IS_ENABLED(CONFIG_RESCTRL_ASSIGN_FIXED) check to handle Arm64

(needs imperative)

> platforms. On x86, CONFIG_RESCTRL_ASSIGN_FIXED is not defined, whereas on
> Arm64, it is. As a result, for MPAM, the file would be either:

CONFIG_RESCTRL_ASSIGN_FIXED does not yet exist anywhere so this motivation needs
to provide stronger support for why it is used before it exists. There is a precedent
here with RESCTRL_RMID_DEPENDS_ON_CLOSID already used while it does not yet
appear in a Kconfig file. I would propose that this is motivated by noting
how it is already understood how Arm supports assignable counters this was recommended
by James to prepare for that work. Since this is user interface this
work is done early to ensure user interface is compatible with that upcoming
support. Also set folks at ease that IS_ENABLED() works as expected with a
non-existing config.


> [default]
> or
> [mbm_cntr_assign]
> 
> Signed-off-by: Babu Moger <babu.moger@amd.com>
> ---
> 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.
> 
> ???END
> ---
>  Documentation/arch/x86/resctrl.rst     | 27 +++++++++++++++++++
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 37 ++++++++++++++++++++++++++
>  2 files changed, 64 insertions(+)
> 
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index fb90f08e564e..bb96b44019fe 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -257,6 +257,33 @@ 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":
> +	Reports the list of monitoring modes supported. The enclosed brackets
> +	indicate which mode is enabled.
> +	::
> +
> +	  # cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> +	  [mbm_cntr_assign]
> +	  default
> +
> +	"mbm_cntr_assign":
> +
> +	In mbm_cntr_assign mode, a monitoring event can only accumulate data
> +	while it is backed by a hardware counter. The user-space is able to
> +	specify which of the events in CTRL_MON or MON groups should have a
> +	counter assigned using the "mbm_assign_control" file. The number of

"mbm_assign_control" no longer exist.

> +	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_cntr_assign mode, if supported, to prevent
> +	the hardware from resetting counters between reads. This can result in

"from resetting counters" -> "from re-allocating counters"?

> +	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/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 17de38e26f94..626be6becca7 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -882,6 +882,36 @@ static int rdtgroup_rmid_show(struct kernfs_open_file *of,
>  	return ret;
>  }
>  
> +static int resctrl_mbm_assign_mode_show(struct kernfs_open_file *of,
> +					struct seq_file *s, void *v)
> +{
> +	struct rdt_resource *r = of->kn->parent->priv;
> +	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_cntr_assign]\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_cntr_assign\n");
> +		}
> +	} else {
> +		seq_puts(s, "[default]\n");
> +	}
> +
> +	mutex_unlock(&rdtgroup_mutex);
> +
> +	return 0;
> +}
> +
>  #ifdef CONFIG_PROC_CPU_RESCTRL
>  
>  /*
> @@ -1908,6 +1938,13 @@ static struct rftype res_common_files[] = {
>  		.seq_show	= mbm_local_bytes_config_show,
>  		.write		= mbm_local_bytes_config_write,
>  	},
> +	{
> +		.name		= "mbm_assign_mode",
> +		.mode		= 0444,
> +		.kf_ops		= &rdtgroup_kf_single_ops,
> +		.seq_show	= resctrl_mbm_assign_mode_show,
> +		.fflags		= RFTYPE_MON_INFO,

Needs a RFTYPE_RES_CACHE?

> +	},
>  	{
>  		.name		= "cpus",
>  		.mode		= 0644,

Reinette