fs/resctrl: Document that automatic counter assignment is best effort

When using automatic counter assignment it's useful for a user to know
which counters they can expect to be assigned on group creation.

Document that automatic counter assignment is best effort and how to
discover any assignment failures.

Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Babu Moger <babu.moger@amd.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Link: https://lore.kernel.org/20260506082855.3694761-1-ben.horgan@arm.com

diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
index 2898a51e6f4be382b66bc541f6f2a7625950da8f..b388e9193896ee2c0f57aa4d7a10712c790d1bad 100644 (file)
--- a/Documentation/filesystems/resctrl.rst
+++ b/Documentation/filesystems/resctrl.rst
@@ -479,6 +479,12 @@ with the following files:
        "1":
                Auto assignment is enabled.
 
+       Automatic counter assignment is done with best effort. If auto
+       assignment is enabled but there are not enough available counters then
+       monitor group creation could succeed while one or more events belonging
+       to the group may not have a counter assigned in all domains. Consult
+       mbm_L3_assignments for counter assignment states of the new groups.
+
        Example::
 
          # echo 0 > /sys/fs/resctrl/info/L3_MON/mbm_assign_on_mkdir