linux-kernel - [tip: x86/cache] x86/resctrl: Avoid redundant counter read in __mon_event

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Date:   Wed, 15 Mar 2023 23:36:18 -0000
From:   "tip-bot2 for Peter Newman" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Peter Newman <peternewman@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Babu Moger <babu.moger@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/cache] x86/resctrl: Avoid redundant counter read in
 __mon_event_count()

The following commit has been merged into the x86/cache branch of tip:

Commit-ID:     322b72e0fd10101f2da8985b31b4af70f184bf79
Gitweb:        https://git.kernel.org/tip/322b72e0fd10101f2da8985b31b4af70f184bf79
Author:        Peter Newman <peternewman@...gle.com>
AuthorDate:    Tue, 20 Dec 2022 17:41:32 +01:00
Committer:     Dave Hansen <dave.hansen@...ux.intel.com>
CommitterDate: Wed, 15 Mar 2023 15:44:15 -07:00

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@...gle.com>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>
Tested-by: Babu Moger <babu.moger@....com>
Link: https://lore.kernel.org/all/20221220164132.443083-2-peternewman%40google.com
---
 arch/x86/kernel/cpu/resctrl/monitor.c | 43 +++++++++++---------------
 1 file changed, 19 insertions(+), 24 deletions(-)

diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index 7fe5148..2095241 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/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;