memcg: increase MEMCG_CHARGE_BATCH to 64

For several years, MEMCG_CHARGE_BATCH was kept at 32 but with bigger
machines and the network intensive workloads requiring througput in Gbps,
32 is too small and makes the memcg charging path a bottleneck.  For now,
increase it to 64 for easy acceptance to 6.0.  We will need to revisit
this in future for ever increasing demand of higher performance.

Please note that the memcg charge path drain the per-cpu memcg charge
stock, so there should not be any oom behavior change.  Though it does
have impact on rstat flushing and high limit reclaim backoff.

To evaluate the impact of this optimization, on a 72 CPUs machine, we
ran the following workload in a three level of cgroup hierarchy.

 $ 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              17064.7 Mbps (62.7% improvement)

With the patch, the throughput improved by 62.7%.

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: Muchun Song <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: "Michal Koutný" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 4 insertions, 3 deletions

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 6257867fbf95..a2461f9a8738 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -354,10 +354,11 @@ struct mem_cgroup {
 };
 
 /*
- * size of first charge trial. "32" comes from vmscan.c's magic value.
- * TODO: maybe necessary to use big numbers in big irons.
+ * size of first charge trial.
+ * TODO: maybe necessary to use big numbers in big irons or dynamic based of the
+ * workload.
  */
-#define MEMCG_CHARGE_BATCH 32U
+#define MEMCG_CHARGE_BATCH 64U
 
 extern struct mem_cgroup *root_mem_cgroup;