Re: [PATCH v12 25/25] x86,fs/resctrl: Document telemetry mount timing caveat

[Reinette Chatre]

Hi Tony,

On 9/16/26 4:13 PM, Tony Luck wrote:

> diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
> index e4b66af55ffb..91a2ab0fa027 100644
> --- a/Documentation/filesystems/resctrl.rst
> +++ b/Documentation/filesystems/resctrl.rst
> @@ -630,7 +630,7 @@ When monitoring is enabled all MON groups will also contain:
>  	each instance of an L3 cache. Each directory contains files for the enabled
>  	L3 events (e.g. "llc_occupancy", "mbm_total_bytes", and "mbm_local_bytes").
>  
> -	If telemetry monitoring is enabled, there will be a "mon_PERF_PKG_YY"
> +	If telemetry monitoring is enabled [#]_, there will be a "mon_PERF_PKG_YY"
>  	directory for each physical processor package. Each directory contains
>  	files for the enabled telemetry events (e.g. "core_energy". "activity",
>  	"uops_retired", etc.)
> @@ -669,6 +669,10 @@ When monitoring is enabled all MON groups will also contain:
>  	returned if the MBM event does not have an assigned counter in the
>  	CTRL_MON group nor in any of its associated MON groups.
>  
> +.. [#] Telemetry features are enumerated asynchronously by the PMT driver, so
> +   an automatic mount of resctrl from ``/etc/fstab`` at boot may not enable
> +   them. See `Mounting resctrl with telemetry`_ below.
> +
>  "mon_hw_id":
>  	Available only with debug option. The identifier used by hardware
>  	for the monitor group. On x86 this is the RMID.
> @@ -1898,6 +1902,18 @@ m. Unmount the resctrl filesystem.
>  
>    # umount /sys/fs/resctrl/
>  
> +Mounting resctrl with telemetry
> +===============================
> +
> +Telemetry features (e.g. the ``mon_PERF_PKG_YY`` events) are enumerated
> +asynchronously by the PMT driver. If resctrl is mounted before that
> +enumeration completes (for example, when mounted automatically from
> +``/etc/fstab`` early in boot) the telemetry features will not be available
> +at mount time and will therefore not be enabled in the mounted instance.

Above paragraph seems practically identical to the footnote content. The only
new information added by this section seems to be the sentence below. Could
this be simplified by dropping this section and just replacing the footnote's
"See `Mounting resctrl with telemetry`_ below." with the sentence below?

> +
> +To avoid this race, defer the mount until after the ``pmt_telemetry`` module
> +has loaded.
> +
>  Intel RDT Errata
>  ================
>  

Reinette