diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
index e38d7fc4f427..0da44cad28fd 100644
--- a/arch/x86/kernel/cpu/resctrl/core.c
+++ b/arch/x86/kernel/cpu/resctrl/core.c
@@ -110,12 +110,23 @@ struct rdt_hw_resource rdt_resources_all[RDT_NUM_RESOURCES] = {
 	},
 };
 
+/**
+ * resctrl_arch_system_num_rmid_idx - Compute number of supported RMIDs
+ *				      (minimum across all mon_capable resource)
+ *
+ * Return: Number of supported RMIDs at time of call. Note that mount time
+ * enumeration of resources may reduce the number.
+ */
 u32 resctrl_arch_system_num_rmid_idx(void)
 {
-	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
+	u32 num_rmids = U32_MAX;
+	struct rdt_resource *r;
+
+	for_each_mon_capable_rdt_resource(r)
+		num_rmids = min(num_rmids, r->mon.num_rmid);
 
 	/* RMID are independent numbers for x86. num_rmid_idx == num_rmid */
-	return r->mon.num_rmid;
+	return num_rmids == U32_MAX ? 0 : num_rmids;
 }
 
 struct rdt_resource *resctrl_arch_get_resource(enum resctrl_res_level l)
diff --git a/fs/resctrl/rdtgroup.c b/fs/resctrl/rdtgroup.c
index 90c4a199a288..666cac7661ce 100644
--- a/fs/resctrl/rdtgroup.c
+++ b/fs/resctrl/rdtgroup.c
@@ -4353,6 +4353,12 @@ void resctrl_offline_mon_domain(struct rdt_resource *r, struct rdt_domain_hdr *h
  * During boot this may be called before global allocations have been made by
  * resctrl_l3_mon_resource_init().
  *
+ * Called during CPU online that may run as soon as CPU online callbacks
+ * are set up during resctrl initialization. The number of supported RMIDs
+ * may be reduced if additional mon_capable resources are enumerated
+ * at mount time. This means the rdt_l3_mon_domain::mbm_states[] and
+ * rdt_l3_mon_domain::rmid_busy_llc allocations may be larger than needed.
+ *
  * Return: 0 for success, or -ENOMEM.
  */
 static int domain_setup_l3_mon_state(struct rdt_resource *r, struct rdt_l3_mon_domain *d)