[PATCH 1/3] mm: page_counter: remove unneeded atomic ops for low/min

[Shakeel Butt]

For cgroups using low or min protections, the function
propagate_protected_usage() was doing an atomic xchg() operation
irrespectively. It only needs to do that operation if the new value of
protection is different from older one. This patch does that.

To evaluate the impact of this optimization, on a 72 CPUs machine, we
ran the following workload in a three level of cgroup hierarchy with top
level having min and low setup appropriately. More specifically
memory.min equal to size of netperf binary and memory.low double of
that.

 $ netserver -6
 # 36 instances of netperf with following params
 $ netperf -6 -H ::1 -l 60 -t TCP_SENDFILE -- -m 10K

Results (average throughput of netperf):
Without (6.0-rc1)	10482.7 Mbps
With patch		14542.5 Mbps (38.7% improvement)

With the patch, the throughput improved by 38.7%

Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
---
 mm/page_counter.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/mm/page_counter.c b/mm/page_counter.c
index eb156ff5d603..47711aa28161 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -17,24 +17,23 @@ static void propagate_protected_usage(struct page_counter *c,
 				      unsigned long usage)
 {
 	unsigned long protected, old_protected;
-	unsigned long low, min;
 	long delta;
 
 	if (!c->parent)
 		return;
 
-	min = READ_ONCE(c->min);
-	if (min || atomic_long_read(&c->min_usage)) {
-		protected = min(usage, min);
+	protected = min(usage, READ_ONCE(c->min));
+	old_protected = atomic_long_read(&c->min_usage);
+	if (protected != old_protected) {
 		old_protected = atomic_long_xchg(&c->min_usage, protected);
 		delta = protected - old_protected;
 		if (delta)
 			atomic_long_add(delta, &c->parent->children_min_usage);
 	}
 
-	low = READ_ONCE(c->low);
-	if (low || atomic_long_read(&c->low_usage)) {
-		protected = min(usage, low);
+	protected = min(usage, READ_ONCE(c->low));
+	old_protected = atomic_long_read(&c->low_usage);
+	if (protected != old_protected) {
 		old_protected = atomic_long_xchg(&c->low_usage, protected);
 		delta = protected - old_protected;
 		if (delta)
-- 
2.37.1.595.g718a3a8f04-goog