Re: [PATCH v11 23/23] x86/resctrl: Document telemetry mount timing caveat

[Reinette Chatre]

Hi Tony,

On 8/31/26 10:44 AM, Tony Luck wrote:
> @@ -1898,6 +1902,60 @@ 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.
> +
> +To avoid this race, defer the mount until after the ``pmt_telemetry`` module
> +has loaded. One way to do this is with a udev rule that triggers a systemd
> +service when the module appears. A tmpfs mount onto ``/sys/fs/resctrl`` is
> +needed to prevent daemon tasks from obtaining references to the resctrl
> +file system in the other namespaces that systemd uses during startup.
> +

Getting the example right seems to be a challenge. I will not be able to support
anybody responding with issues when trying out these instructions and
I would prefer to leave such details to the experts. Could the text starting
with "One way to do this ..." be removed instead and just rely on the high level
guidance that resctrl should be mounted after pmt_telemetry module is loaded? 

Reinette