[PATCH v14 24/24] x86,fs/resctrl: Document telemetry mount timing caveat

[Tony Luck]

resctrl enumerates telemetry events each time it is mounted, using
functions that the pmt_telemetry driver registers when it loads. The
driver can be built as a module, which is typically loaded by udev during
boot.

An automatic mount of resctrl from /etc/fstab early in boot can run
before pmt_telemetry has loaded. Telemetry events are then not enabled
in that mount, with no indication why.

Document the race, how to avoid it by mounting resctrl after
pmt_telemetry has loaded, and that unmounting and mounting resctrl again
enables the telemetry events.

Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
---
v14:
	Updated commit message. Include unmount/remount as solution
	if early mount did not include AET.
---
 Documentation/filesystems/resctrl.rst | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
index b52795e03303..66ffb625aac0 100644
--- a/Documentation/filesystems/resctrl.rst
+++ b/Documentation/filesystems/resctrl.rst
@@ -629,7 +629,7 @@ 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.)
@@ -668,6 +668,12 @@ 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. To avoid this race, defer the mount until after the ``pmt_telemetry``
+   module has loaded. Unmounting and remounting the resctrl file system will
+   fix the issue.
+
 "mon_hw_id":
 	Available only with debug option. The identifier used by hardware
 	for the monitor group. On x86 this is the RMID.
-- 
2.56.0