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

[Tony Luck <tony.luck@intel.com>]

The PMT driver enumerates telemetry features asynchronously, so an
automatic mount of resctrl from /etc/fstab early in boot may occur before
those features are available, resulting in them not being enabled in the
mounted instance.

Add a footnote to the 'If telemetry monitoring is enabled' sentence
pointing readers to a new "Mounting resctrl with telemetry" section. That
section explains the race.

Signed-off-by: Tony Luck <tony.luck@intel.com>
---
v12:
	Drop problematic examples of systemd/udev rules.
---
 Documentation/filesystems/resctrl.rst | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

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.
+
+To avoid this race, defer the mount until after the ``pmt_telemetry`` module
+has loaded.
+
 Intel RDT Errata
 ================
 
-- 
2.55.0