linux-kernel - [PATCH] mm/vmstat: Use this_cpu_try_cmpxchg in mod_{zone,node}

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Message-ID: <20230904150917.8318-1-ubizjak@gmail.com>
Date:   Mon,  4 Sep 2023 17:08:49 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] mm/vmstat: Use this_cpu_try_cmpxchg in mod_{zone,node}_state

Use this_cpu_try_cmpxchg instead of this_cpu_cmpxchg (*ptr, old, new) == old
in mod_zone_state and mod_node_state.  x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg (and
related move instruction in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails. There is no need to re-read the value in the loop.

No functional change intended.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
---
 mm/vmstat.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 00e81e99c6ee..894e4c88d241 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -559,8 +559,10 @@ static inline void mod_zone_state(struct zone *zone,
 {
 	struct per_cpu_zonestat __percpu *pcp = zone->per_cpu_zonestats;
 	s8 __percpu *p = pcp->vm_stat_diff + item;
-	long o, n, t, z;
+	long n, t, z;
+	s8 o;
 
+	o = this_cpu_read(*p);
 	do {
 		z = 0;  /* overflow to zone counters */
 
@@ -576,8 +578,7 @@ static inline void mod_zone_state(struct zone *zone,
 		 */
 		t = this_cpu_read(pcp->stat_threshold);
 
-		o = this_cpu_read(*p);
-		n = delta + o;
+		n = delta + (long)o;
 
 		if (abs(n) > t) {
 			int os = overstep_mode * (t >> 1) ;
@@ -586,7 +587,7 @@ static inline void mod_zone_state(struct zone *zone,
 			z = n + os;
 			n = -os;
 		}
-	} while (this_cpu_cmpxchg(*p, o, n) != o);
+	} while (!this_cpu_try_cmpxchg(*p, &o, n));
 
 	if (z)
 		zone_page_state_add(z, zone, item);
@@ -616,7 +617,8 @@ static inline void mod_node_state(struct pglist_data *pgdat,
 {
 	struct per_cpu_nodestat __percpu *pcp = pgdat->per_cpu_nodestats;
 	s8 __percpu *p = pcp->vm_node_stat_diff + item;
-	long o, n, t, z;
+	long n, t, z;
+	s8 o;
 
 	if (vmstat_item_in_bytes(item)) {
 		/*
@@ -629,6 +631,7 @@ static inline void mod_node_state(struct pglist_data *pgdat,
 		delta >>= PAGE_SHIFT;
 	}
 
+	o = this_cpu_read(*p);
 	do {
 		z = 0;  /* overflow to node counters */
 
@@ -644,8 +647,7 @@ static inline void mod_node_state(struct pglist_data *pgdat,
 		 */
 		t = this_cpu_read(pcp->stat_threshold);
 
-		o = this_cpu_read(*p);
-		n = delta + o;
+		n = delta + (long)o;
 
 		if (abs(n) > t) {
 			int os = overstep_mode * (t >> 1) ;
@@ -654,7 +656,7 @@ static inline void mod_node_state(struct pglist_data *pgdat,
 			z = n + os;
 			n = -os;
 		}
-	} while (this_cpu_cmpxchg(*p, o, n) != o);
+	} while (!this_cpu_try_cmpxchg(*p, &o, n));
 
 	if (z)
 		node_page_state_add(z, pgdat, item);
-- 
2.41.0