Re: [PATCH v5 07/20] x86/resctrl: Introduce the interface to display monitor mode

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

Hi Babu,

On 7/3/24 2:48 PM, Babu Moger wrote:
> The ABMC feature provides an option to the user to assign a hardware
> counter to an RMID and monitor the bandwidth as long as it is assigned.
> ABMC mode is enabled by default when supported. System can be one mode
> at a time (Legacy monitor mode or ABMC mode).
> 
> Provide an interface to display the monitor mode on the system.
>      $cat /sys/fs/resctrl/info/L3_MON/mbm_mode
>      [abmc]
>      legacy

<insert snippet about what happens when user switches from one mode
to another>

> 
> Signed-off-by: Babu Moger <babu.moger@amd.com>
> ---
> 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/arch/x86/resctrl.rst     | 30 ++++++++++++++++++++++++++
>   arch/x86/kernel/cpu/resctrl/monitor.c  |  2 ++
>   arch/x86/kernel/cpu/resctrl/rdtgroup.c | 26 ++++++++++++++++++++++
>   3 files changed, 58 insertions(+)
> 
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index 30586728a4cd..108e494fd7cc 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -257,6 +257,36 @@ with the following files:
>   	    # cat /sys/fs/resctrl/info/L3_MON/mbm_local_bytes_config
>   	    0=0x30;1=0x30;3=0x15;4=0x15
>   
> +"mbm_mode":
> +	Reports the list of assignable monitoring features supported. The
> +	enclosed brackets indicate which feature is enabled.
> +	::
> +
> +	  cat /sys/fs/resctrl/info/L3_MON/mbm_mode
> +	  [abmc]
> +	  legacy
> +

"mbm_cntr" mode can be documented here with the details on how AMD's ABMC is
one example of how it may be implemented on a system.

> +	The bandwidth monitoring feature on AMD system only guarantees that
> +	RMIDs currently assigned to a processor will be tracked by hardware.
> +	The counters of any other RMIDs which are no longer being tracked
> +	will be reset to zero. The MBM event counters return "Unavailable"
> +	for the RMIDs that are not tracked by hardware. So, there can be
> +	only limited number of groups that can give guaranteed monitoring
> +	numbers. With ever changing configurations there is no way to
> +	definitely know which of these groups are being tracked for certain
> +	point of time. Users do not have the option to monitor a group or
> +	set of groups for certain period of time without worrying about
> +	RMID being reset in between.
> +
> +	The ABMC feature provides an option to the user to assign a
> +	hardware counter to an RMID and monitor the bandwidth as long as
> +	it is assigned. The assigned RMID will be tracked by the hardware
> +	until the user unassigns it manually. There is no need to worry
> +	about counters being reset during this period.
> +
> +	Without ABMC enabled, monitoring will work in "legacy" mode
> +	without assignment option.

Let "legacy" be a distinct mode, instead of an alternative to ABMC.

> +
>   "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/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
> index 12793762ca24..6c4cb36b4b50 100644
> --- a/arch/x86/kernel/cpu/resctrl/monitor.c
> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c
> @@ -1245,6 +1245,8 @@ int __init rdt_get_mon_l3_config(struct rdt_resource *r)
>   		}
>   	}
>   
> +	resctrl_file_fflags_init("mbm_mode", RFTYPE_MON_INFO);
> +

Is this special flag assignment necessary? With file always visible I think it
can just be initialized in res_common_files below with the flag already assigned?

>   	l3_mon_evt_init(r);
>   
>   	r->mon_capable = true;
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index 471fc0dbd7c3..3988d7b86817 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -845,6 +845,26 @@ static int rdtgroup_rmid_show(struct kernfs_open_file *of,
>   	return ret;
>   }
>   
> +static int rdtgroup_mbm_mode_show(struct kernfs_open_file *of,
> +				  struct seq_file *s, void *v)
> +{
> +	struct rdt_resource *r = of->kn->parent->priv;
> +
> +	if (r->mon.abmc_capable) {
> +		if (resctrl_arch_get_abmc_enabled()) {
> +			seq_puts(s, "[abmc]\n");
> +			seq_puts(s, "legacy\n");
> +		} else {
> +			seq_puts(s, "abmc\n");
> +			seq_puts(s, "[legacy]\n");
> +		}
> +	} else {
> +		seq_puts(s, "[legacy]\n");
> +	}
> +
> +	return 0;
> +}
> +
>   #ifdef CONFIG_PROC_CPU_RESCTRL
>   
>   /*
> @@ -1901,6 +1921,12 @@ static struct rftype res_common_files[] = {
>   		.seq_show	= mbm_local_bytes_config_show,
>   		.write		= mbm_local_bytes_config_write,
>   	},
> +	{
> +		.name		= "mbm_mode",
> +		.mode		= 0444,
> +		.kf_ops		= &rdtgroup_kf_single_ops,
> +		.seq_show	= rdtgroup_mbm_mode_show,
> +	},
>   	{
>   		.name		= "cpus",
>   		.mode		= 0644,

Reinette