mm: page_counter: remove unneeded atomic ops for low/min

Patch series "memcg: optimize charge codepath", v2.

Recently Linux networking stack has moved from a very old per socket
pre-charge caching to per-cpu caching to avoid pre-charge fragmentation
and unwarranted OOMs.  One impact of this change is that for network
traffic workloads, memcg charging codepath can become a bottleneck.  The
kernel test robot has also reported this regression[1].  This patch series
tries to improve the memcg charging for such workloads.

This patch series implement three optimizations:
(A) Reduce atomic ops in page counter update path.
(B) Change layout of struct page_counter to eliminate false sharing
    between usage and high.
(C) Increase the memcg charge batch to 64.

To evaluate the impact of these optimizations, on a 72 CPUs machine, we
ran the following workload in root memcg and then compared with scenario
where the workload is run in a three level of cgroup hierarchy with top
level having min and low setup appropriately.

 $ 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):
1. root memcg		21694.8 Mbps
2. 6.0-rc1		10482.7 Mbps (-51.6%)
3. 6.0-rc1 + (A)	14542.5 Mbps (-32.9%)
4. 6.0-rc1 + (B)	12413.7 Mbps (-42.7%)
5. 6.0-rc1 + (C)	17063.7 Mbps (-21.3%)
6. 6.0-rc1 + (A+B+C)	20120.3 Mbps (-7.2%)

With all three optimizations, the memcg overhead of this workload has
been reduced from 51.6% to just 7.2%.

[1] https://lore.kernel.org/linux-mm/20220619150456.GB34471@xsang-OptiPlex-9020/


This patch (of 3):

For cgroups using low or min protections, the function
propagate_protected_usage() was doing an atomic xchg() operation
irrespectively.  We can optimize out this atomic operation for one
specific scenario where the workload is using the protection (i.e.  min >
0) and the usage is above the protection (i.e.  usage > min).

This scenario is actually very common where the users want a part of their
workload to be protected against the external reclaim.  Though this
optimization does introduce a race when the usage is around the protection
and concurrent charges and uncharged trip it over or under the protection.
In such cases, we might see lower effective protection but the subsequent
charge/uncharge will correct it.

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 to see if this optimization is
effective for the mentioned case.

 $ 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%

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Shakeel Butt <[email protected]>
Reported-by: kernel test robot <[email protected]>
Acked-by: Soheil Hassas Yeganeh <[email protected]>
Reviewed-by: Feng Tang <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: "Michal Koutný" <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Oliver Sang <[email protected]>
Cc: Soheil Hassas Yeganeh <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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)