mm: page_counter: re-layout structure to reduce false sharing

When checking a memory cgroup related performance regression [1], from the
perf c2c profiling data, we found high false sharing for accessing 'usage'
and 'parent'.

On 64 bit system, the 'usage' and 'parent' are close to each other, and
easy to be in one cacheline (for cacheline size == 64+ B). 'usage' is
usally written, while 'parent' is usually read as the cgroup's
hierarchical counting nature.

So move the 'parent' to the end of the structure to make sure they
are in different cache lines.

Following are some performance data with the patch, against v5.11-rc1. [
In the data, A means a platform with 2 sockets 48C/96T, B is a platform of
4 sockests 72C/144T, and if a %stddev will be shown bigger than 2%,
P100/P50 means number of test tasks equals to 100%/50% of nr_cpu]

will-it-scale/malloc1
---------------------
v5.11-rc1 v5.11-rc1+patch

A-P100 15782 ± 2% -0.1% 15765 ± 3% will-it-scale.per_process_ops
A-P50 21511 +8.9% 23432 will-it-scale.per_process_ops
B-P100 9155 +2.2% 9357 will-it-scale.per_process_ops
B-P50 10967 +7.1% 11751 ± 2% will-it-scale.per_process_ops

will-it-scale/pagefault2
------------------------
v5.11-rc1 v5.11-rc1+patch

A-P100 79028 +3.0% 81411 will-it-scale.per_process_ops
A-P50 183960 ± 2% +4.4% 192078 ± 2% will-it-scale.per_process_ops
B-P100 85966 +9.9% 94467 ± 3% will-it-scale.per_process_ops
B-P50 198195 +9.8% 217526 will-it-scale.per_process_ops

fio (4k/1M is block size)
-------------------------
v5.11-rc1 v5.11-rc1+patch

A-P50-r-4k 16881 ± 2% +1.2% 17081 ± 2% fio.read_bw_MBps
A-P50-w-4k 3931 +4.5% 4111 ± 2% fio.write_bw_MBps
A-P50-r-1M 15178 -0.2% 15154 fio.read_bw_MBps
A-P50-w-1M 3924 +0.1% 3929 fio.write_bw_MBps

[1].https://lore.kernel.org/lkml/20201102091543.GM31092@shao2-debian/

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Feng Tang <[e-mail]>
Reviewed-by: Roman Gushchin <[e-mail]>
Reviewed-by: Shakeel Butt <[e-mail]>
Acked-by: Johannes Weiner <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/page_counter.h

···
 	unsigned long low;
 	unsigned long high;
 	unsigned long max;
-	struct page_counter *parent;
 
 	/* effective memory.min and memory.min usage tracking */
 	unsigned long emin;
···
 	/* legacy */
 	unsigned long watermark;
 	unsigned long failcnt;
+
+	/*
+	 * 'parent' is placed here to be far from 'usage' to reduce
+	 * cache false sharing, as 'usage' is written mostly while
+	 * parent is frequently read for cgroup's hierarchical
+	 * counting nature.
+	 */
+	struct page_counter *parent;
 };
 
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