x86/resctrl: Avoid redundant counter read in __mon_event_count()

__mon_event_count() does the per-RMID, per-domain work for
user-initiated event count reads and the initialization of new monitor
groups.

In the initialization case, after resctrl_arch_reset_rmid() calls
__rmid_read() to record an initial count for a new monitor group, it
immediately calls resctrl_arch_rmid_read(). This re-read of the hardware
counter is unnecessary and the following computations are ignored by the
caller during initialization.

Following return from resctrl_arch_reset_rmid(), just clear the
mbm_state and return. This involves moving the mbm_state lookup into the
rr->first case, as it's not needed for regular event count reads: the
QOS_L3_OCCUP_EVENT_ID case was redundant with the accumulating logic at
the end of the function.

Signed-off-by: Peter Newman <peternewman@google.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Link: https://lore.kernel.org/all/20221220164132.443083-2-peternewman%40google.com

Author: Peter Newman

arch/x86/kernel/cpu/resctrl/monitor.c

@@ -383,41 +383,36 @@ void free_rmid(u32 rmid)
 		list_add_tail(&entry->list, &rmid_free_lru);
 }
 
+static struct mbm_state *get_mbm_state(struct rdt_domain *d, u32 rmid,
+				       enum resctrl_event_id evtid)
+{
+	switch (evtid) {
+	case QOS_L3_MBM_TOTAL_EVENT_ID:
+		return &d->mbm_total[rmid];
+	case QOS_L3_MBM_LOCAL_EVENT_ID:
+		return &d->mbm_local[rmid];
+	default:
+		return NULL;
+	}
+}
+
 static int __mon_event_count(u32 rmid, struct rmid_read *rr)
 {
 	struct mbm_state *m;
 	u64 tval = 0;
 
-	if (rr->first)
+	if (rr->first) {
 		resctrl_arch_reset_rmid(rr->r, rr->d, rmid, rr->evtid);
+		m = get_mbm_state(rr->d, rmid, rr->evtid);
+		if (m)
+			memset(m, 0, sizeof(struct mbm_state));
+		return 0;
+	}
 
 	rr->err = resctrl_arch_rmid_read(rr->r, rr->d, rmid, rr->evtid, &tval);
 	if (rr->err)
 		return rr->err;
 
-	switch (rr->evtid) {
-	case QOS_L3_OCCUP_EVENT_ID:
-		rr->val += tval;
-		return 0;
-	case QOS_L3_MBM_TOTAL_EVENT_ID:
-		m = &rr->d->mbm_total[rmid];
-		break;
-	case QOS_L3_MBM_LOCAL_EVENT_ID:
-		m = &rr->d->mbm_local[rmid];
-		break;
-	default:
-		/*
-		 * Code would never reach here because an invalid
-		 * event id would fail in resctrl_arch_rmid_read().
-		 */
-		return -EINVAL;
-	}
-
-	if (rr->first) {
-		memset(m, 0, sizeof(struct mbm_state));
-		return 0;
-	}
-
 	rr->val += tval;
 
 	return 0;