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/[email protected]
Signed-off-by: Feng Tang <[email protected]>
Reviewed-by: Roman Gushchin <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/page_counter.h

@@ -12,7 +12,6 @@ struct page_counter {
 	unsigned long low;
 	unsigned long high;
 	unsigned long max;
-	struct page_counter *parent;
 
 	/* effective memory.min and memory.min usage tracking */
 	unsigned long emin;
@@ -27,6 +26,14 @@ struct page_counter {
 	/* 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;
 };
 
 #if BITS_PER_LONG == 32