[PATCH v4 2/3] fs/resctrl: Assign counters to existing groups when enabling mbm_event

[Babu Moger <babu.moger@amd.com>]

When the user switches counter assignment mode by writing "mbm_event" to
/sys/fs/resctrl/info/L3_MON/mbm_assign_mode, resctrl frees all assignable
counters, resets per-domain RMID state, and enables counter auto-assignment
exposed to user space as mbm_assign_on_mkdir.

Even though counter auto-assignment is enabled, groups that already exist
at the time of the switch, including the default group created at mount,
are not assigned a counter. All MBM events read "Unassigned" until the user
assigns a counter by hand.

Walk every existing CTRL_MON and its MON children after the reset and
assign counters to their MBM events. Enabling "mbm_event" now leaves the
same per-group state that auto-assignment would have produced. There may be
fewer available counters than MBM events across the existing groups; in
that case stop assignment when no counters remain. Events in the remaining
groups read "Unassigned", matching the behavior of creating a group when no
counters are available. Also revise the rdtgroup_assign_cntrs() function
comments to account for the new callers.

Fixes: 8004ea01cf63 ("fs/resctrl: Introduce the interface to switch between monitor modes")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/8cb66e18e32e4087a9712c1e68ee6da614efe244.1784322818.git.babu.moger%40amd.com
Signed-off-by: Babu Moger <babu.moger@amd.com>
Cc: stable@vger.kernel.org
---
v4: Changelog update (Reinette's input).
    Function comment update for rdtgroup_assign_cntrs().

v3: Changelog update.
    Code comment cleanup.
    Added Cc to stable.
    Added Reported-by and Closes tags.

v2: New patch.
    This patch addresses the Sashiko comment about documentation issue where
    counters are not assigned automatically when mode is switched to mbm_event.
    https://sashiko.dev/#/patchset/8cb66e18e32e4087a9712c1e68ee6da614efe244.1784322818.git.babu.moger%40amd.com
    In fact, it exposed a real issue. When switching to mbm_event mode, existing
    monitoring groups should be assigned counters whenever counters are available.
    This provides a smooth transition between modes and aligns the behavior with
    the existing auto-assignment mechanism.
---
 Documentation/filesystems/resctrl.rst |  7 ++--
 fs/resctrl/monitor.c                  | 47 ++++++++++++++++++++++-----
 2 files changed, 43 insertions(+), 11 deletions(-)

diff --git a/Documentation/filesystems/resctrl.rst b/Documentation/filesystems/resctrl.rst
index b52795e03303..c8507580474a 100644
--- a/Documentation/filesystems/resctrl.rst
+++ b/Documentation/filesystems/resctrl.rst
@@ -370,8 +370,11 @@ with the following files:
 	of counters available is described in the "num_mbm_cntrs" file. Changing the
 	mode may cause all counters on the resource to reset.
 
-	Moving to mbm_event counter assignment mode requires users to assign the counters
-	to the events. Otherwise, the MBM event counters will return 'Unassigned' when read.
+	Moving to mbm_event counter assignment mode enables "mbm_assign_on_mkdir" and
+	assigns counters to the events of all existing monitoring groups, including
+	the default group, while counters remain available. Consult
+	"mbm_L3_assignments" after switching to "mbm_event" mode for counter
+	assignment states of all monitoring groups.
 
 	The mode is beneficial for AMD platforms that support more CTRL_MON
 	and MON groups than available hardware counters. By default, this
diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index 04e4ea612b7c..9e705eafe873 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -1302,14 +1302,17 @@ static int rdtgroup_assign_cntr_event(struct rdt_l3_mon_domain *d, struct rdtgro
 }
 
 /*
- * rdtgroup_assign_cntrs() - Assign counters to MBM events. Called when
- *			     a new group is created.
+ * rdtgroup_assign_cntrs() - Assign counters to enabled MBM events of a
+ *                         resource group
  *
  * Each group can accommodate two counters per domain: one for the total
- * event and one for the local event. Assignments may fail due to the limited
- * number of counters. However, it is not necessary to fail the group creation
- * and thus no failure is returned. Users have the option to modify the
- * counter assignments after the group has been created.
+ * event and one for the local event. Assignments may fail due to the
+ * limited number of counters.
+ *
+ * Intended to be called in auto assignment scenarios ("mbm_assign_on_mkdir"
+ * enabled in "mbm_event" counter assignment mode) where counter assignment
+ * is not required to succeed. Thus no failure is returned. Users can modify
+ * the counter assignments afterwards if needed.
  */
 void rdtgroup_assign_cntrs(struct rdtgroup *rdtgrp)
 {
@@ -1328,6 +1331,22 @@ void rdtgroup_assign_cntrs(struct rdtgroup *rdtgrp)
 					   &mon_event_all[QOS_L3_MBM_LOCAL_EVENT_ID]);
 }
 
+/*
+ * resctrl_assign_cntrs_allrdtgrp() - Assign counters to the MBM events of
+ *				      every existing group.
+ */
+static void resctrl_assign_cntrs_allrdtgrp(void)
+{
+	struct rdtgroup *prgrp, *crgrp;
+
+	list_for_each_entry(prgrp, &rdt_all_groups, rdtgroup_list) {
+		rdtgroup_assign_cntrs(prgrp);
+
+		list_for_each_entry(crgrp, &prgrp->mon.crdtgrp_list, mon.crdtgrp_list)
+			rdtgroup_assign_cntrs(crgrp);
+	}
+}
+
 /*
  * rdtgroup_free_unassign_cntr() - Unassign and reset the counter ID configuration
  * for the event pointed to by @mevt within the domain @d and resctrl group @rdtgrp.
@@ -1601,9 +1620,6 @@ ssize_t resctrl_mbm_assign_mode_write(struct kernfs_open_file *of, char *buf,
 									   (READS_TO_LOCAL_MEM |
 									    READS_TO_LOCAL_S_MEM |
 									    NON_TEMP_WRITE_TO_LOCAL_MEM);
-		/* Enable auto assignment when switching to "mbm_event" mode */
-		if (enable)
-			r->mon.mbm_assign_on_mkdir = true;
 		/*
 		 * Reset all the non-achitectural RMID state and assignable counters.
 		 */
@@ -1611,6 +1627,19 @@ ssize_t resctrl_mbm_assign_mode_write(struct kernfs_open_file *of, char *buf,
 			mbm_cntr_free_all(r, d);
 			resctrl_reset_rmid_all(r, d);
 		}
+
+		/*
+		 * Counters were freed above, so both new groups (via mkdir) and
+		 * the groups that already exist need assignments. Groups created
+		 * while in "default" mode have no counter assigned, including the
+		 * default group created when resctrl is mounted. Assign counters
+		 * to them so that enabling the mode leaves the same assignments
+		 * that mkdir would have made.
+		 */
+		if (enable) {
+			r->mon.mbm_assign_on_mkdir = true;
+			resctrl_assign_cntrs_allrdtgrp();
+		}
 	}
 
 out_unlock:
-- 
2.43.0