Re: [PATCH v11 19/23] x86/resctrl: Introduce the interface to switch between monitor modes

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

Hi Babu,

On 1/22/25 12:20 PM, Babu Moger wrote:
> Resctrl subsystem can support two monitoring modes, 'mbm_cntr_assign' or
> 'default'. In mbm_cntr_assign, 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 interface to switch between mbm_cntr_assign and default modes.
> 
> $ cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> [mbm_cntr_assign]
> default
> 
> To enable the "mbm_cntr_assign" mode:
> $ echo "mbm_cntr_assign" > /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.
> 
> Signed-off-by: Babu Moger <babu.moger@amd.com>
> ---


> ---
>  Documentation/arch/x86/resctrl.rst     | 25 ++++++++++++-
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 50 +++++++++++++++++++++++++-
>  2 files changed, 73 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/arch/x86/resctrl.rst b/Documentation/arch/x86/resctrl.rst
> index 072b15550ff7..5d18c4c8bc48 100644
> --- a/Documentation/arch/x86/resctrl.rst
> +++ b/Documentation/arch/x86/resctrl.rst
> @@ -259,7 +259,10 @@ with the following files:
>  
>  "mbm_assign_mode":
>  	Reports the list of monitoring modes supported. The enclosed brackets
> -	indicate which mode is enabled.
> +	indicate which mode is enabled. The MBM events (mbm_total_bytes and/or
> +	mbm_local_bytes) associated with counters may reset when "mbm_assign_mode"
> +	is changed.
> +
>  	::
>  
>  	  # cat /sys/fs/resctrl/info/L3_MON/mbm_assign_mode
> @@ -275,6 +278,16 @@ with the following files:
>  	available is described in the "num_mbm_cntrs" file. Changing the mode
>  	may cause all counters on a resource to reset.
>  
> +	Moving to mbm_cntr_assign mode require users to assign the counters to
> +	the events. Otherwise, the MBM event counters will return "Unassigned"
> +	when read.

Again ... please be consistent in using single or double quotes for information
returned from file.

> +
> +	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
> @@ -283,6 +296,16 @@ with the following files:
>  	"mbm_total_bytes" or "mbm_local_bytes" will report 'Unavailable' if
>  	there is no counter associated with that event.
>  
> +	* To enable "mbm_cntr_assign" mode:
> +	  ::
> +
> +	    # echo "mbm_cntr_assign" > /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
> +

Please be consistent in the documentation.

To enable "mbm_cntr_assign" mode:
To enable "default" mode:
or
To enable "mbm_cntr_assign" monitoring mode:
To enable "default" monitoring mode:
or 
...?



>  "num_mbm_cntrs":
>  	The number of monitoring counters available for assignment when the
>  	system supports mbm_cntr_assign mode.
> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> index f61f0cd032ef..6922173c4f8f 100644
> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c
> @@ -928,6 +928,53 @@ static int resctrl_available_mbm_cntrs_show(struct kernfs_open_file *of,
>  	return ret;
>  }
>  
> +static ssize_t resctrl_mbm_assign_mode_write(struct kernfs_open_file *of,
> +					     char *buf, size_t nbytes, loff_t off)
> +{
> +	struct rdt_resource *r = of->kn->parent->priv;
> +	int ret = 0;
> +	bool enable;
> +
> +	/* Valid input requires a trailing newline */
> +	if (nbytes == 0 || buf[nbytes - 1] != '\n')
> +		return -EINVAL;
> +
> +	buf[nbytes - 1] = '\0';
> +
> +	cpus_read_lock();
> +	mutex_lock(&rdtgroup_mutex);
> +
> +	rdt_last_cmd_clear();
> +
> +	if (!strcmp(buf, "default")) {
> +		enable = 0;
> +	} else if (!strcmp(buf, "mbm_cntr_assign")) {
> +		if (r->mon.mbm_cntr_assignable) {
> +			enable = 1;
> +		} else {
> +			ret = -EINVAL;
> +			rdt_last_cmd_puts("mbm_cntr_assign mode is not supported\n");
> +			goto write_exit;
> +		}
> +	} else {
> +		ret = -EINVAL;
> +		rdt_last_cmd_puts("Unsupported assign mode\n");
> +		goto write_exit;
> +	}
> +
> +	if (enable != resctrl_arch_mbm_cntr_assign_enabled(r)) {
> +		ret = resctrl_arch_mbm_cntr_assign_set(r, enable);
> +		if (!ret)
> +			mbm_cntr_reset(r);

The following APIs interact with the MBM assignable counters:

mbm_cntr_alloc()
mbm_cntr_get()
mbm_cntr_free()

mbm_cntr_reset() appears to be related but does significantly more
than interact with the MBM assignable counters and that creates a
confusing API.

How about introducing mbm_cntr_free_all() that _only_ releases all
MBM assignable counters and match with mbm_cntr_free() that releases
a single MBM assignable counter? mbm_cntr_free_all() lives with the
other functions operating on MBM assignable counters, thus not
hiding its functionality in other parts of resctrl.

This series open codes reset of non-architectural state in two places,
within mbm_cntr_reset() and within mbm_config_write_domain(). That
can be turned into a new helper that only resets architectural state,
for example resctrl_reset_rmid_all() to match existing
resctrl_arch_reset_rmid_all().

resctrl_arch_mbm_cntr_assign_set() can also reset any architectural
state leaving mbm_cntr_free_all() and resctrl_reset_rmid_all() to be called
here and from within mbm_config_write_domain().

What do you think?

> +	}
> +
> +write_exit:
> +	mutex_unlock(&rdtgroup_mutex);
> +	cpus_read_unlock();
> +
> +	return ret ?: nbytes;
> +}
> +
>  #ifdef CONFIG_PROC_CPU_RESCTRL
>  
>  /*
> @@ -1945,9 +1992,10 @@ static struct rftype res_common_files[] = {
>  	},
>  	{
>  		.name		= "mbm_assign_mode",
> -		.mode		= 0444,
> +		.mode		= 0644,
>  		.kf_ops		= &rdtgroup_kf_single_ops,
>  		.seq_show	= resctrl_mbm_assign_mode_show,
> +		.write		= resctrl_mbm_assign_mode_write,
>  		.fflags		= RFTYPE_MON_INFO,
>  	},
>  	{

Reinette