[PATCH 5/6] memcg: group the read-mostly fields of struct mem_cgroup

[Shakeel Butt]

These fields are set when the memcg is created and only read after
that:

  vmstats_percpu   read on every stat update
  vmstats
  zswap_max, zswap_writeback
  private_id       read on every eviction and refault
  kmemcg_id        read on every list_lru lookup
  oom_group

Put them in one cache line group at the end of the struct, right
before nodeinfo[]. nodeinfo[] is read-mostly too but it is a flexible
array, so it cannot sit inside a group. The group ends without padding
so the two share a line.

This also drops the ____cacheline_aligned_in_smp on vmstats_percpu
added by commit c1afbd5de131 ("mm/memcontrol: avoid false sharing
between vmstats and events"). That only aligned the start of the
field. cgwb_domain followed it on the same line and is written on
every writeout completion. A group boundary covers both sides.

No functional change.

Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
---
 include/linux/memcontrol.h | 62 +++++++++++++++++++++-----------------
 mm/memcontrol.c            | 17 +++++++++++
 2 files changed, 52 insertions(+), 27 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 635929a1f13b..d0f3458f9250 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -183,9 +183,6 @@ struct obj_cgroup {
 struct mem_cgroup {
 	struct cgroup_subsys_state css;
 
-	/* Private memcg ID. Used to ID objects that outlive the cgroup */
-	int private_id;
-
 	/* Accounted resources */
 	struct page_counter memory;		/* Both v1 & v2 */
 
@@ -259,30 +256,6 @@ struct mem_cgroup {
 #endif
 	__cacheline_group_end_aligned(memcg_cold);
 
-#ifdef CONFIG_ZSWAP
-	unsigned long zswap_max;
-
-	/*
-	 * Prevent pages from this memcg from being written back from zswap to
-	 * swap, and from being swapped out on zswap store failures.
-	 */
-	bool zswap_writeback;
-#endif
-
-	/*
-	 * Should the OOM killer kill all belonging tasks, had it kill one?
-	 */
-	bool oom_group;
-
-	/* memory.stat */
-	struct memcg_vmstats	*vmstats;
-
-	int kmemcg_id;
-
-	/* Keep the hot per-CPU stats pointer away from memory event counters. */
-	struct memcg_vmstats_percpu __percpu *vmstats_percpu
-		____cacheline_aligned_in_smp;
-
 #ifdef CONFIG_MEMCG_V1
 	/* v1 only. Not grouped: v1 is legacy, sorting it is not worth it. */
 
@@ -322,6 +295,41 @@ struct mem_cgroup {
 	int swappiness;
 #endif /* CONFIG_MEMCG_V1 */
 
+	/*
+	 * Set when the memcg is created and cleared when it is offlined.
+	 * Never written on a hot path.
+	 */
+	__cacheline_group_begin_aligned(memcg_read_mostly);
+	/* Read on every stat update */
+	struct memcg_vmstats_percpu __percpu *vmstats_percpu;
+
+	/* memory.stat */
+	struct memcg_vmstats	*vmstats;
+
+#ifdef CONFIG_ZSWAP
+	unsigned long zswap_max;
+#endif
+
+	/* Private memcg ID. Used to ID objects that outlive the cgroup */
+	int private_id;
+
+	int kmemcg_id;
+
+	/*
+	 * Should the OOM killer kill all belonging tasks, had it kill one?
+	 */
+	bool oom_group;
+
+#ifdef CONFIG_ZSWAP
+	/*
+	 * Prevent pages from this memcg from being written back from zswap to
+	 * swap, and from being swapped out on zswap store failures.
+	 */
+	bool zswap_writeback;
+#endif
+	/* Not padded: nodeinfo[] is read-mostly too, let it share the line. */
+	__cacheline_group_end(memcg_read_mostly);
+
 	struct mem_cgroup_per_node *nodeinfo[];
 };
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b2cc82c936ed..4a5a30439a03 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5779,6 +5779,23 @@ static void __init memcg_struct_check(void)
 	CACHELINE_ASSERT_GROUP_MEMBER(struct mem_cgroup, memcg_cold,
 				      mm_list);
 #endif
+
+	CACHELINE_ASSERT_GROUP_MEMBER(struct mem_cgroup, memcg_read_mostly,
+				      vmstats_percpu);
+	CACHELINE_ASSERT_GROUP_MEMBER(struct mem_cgroup, memcg_read_mostly,
+				      vmstats);
+#ifdef CONFIG_ZSWAP
+	CACHELINE_ASSERT_GROUP_MEMBER(struct mem_cgroup, memcg_read_mostly,
+				      zswap_max);
+	CACHELINE_ASSERT_GROUP_MEMBER(struct mem_cgroup, memcg_read_mostly,
+				      zswap_writeback);
+#endif
+	CACHELINE_ASSERT_GROUP_MEMBER(struct mem_cgroup, memcg_read_mostly,
+				      private_id);
+	CACHELINE_ASSERT_GROUP_MEMBER(struct mem_cgroup, memcg_read_mostly,
+				      kmemcg_id);
+	CACHELINE_ASSERT_GROUP_MEMBER(struct mem_cgroup, memcg_read_mostly,
+				      oom_group);
 }
 
 int __init mem_cgroup_init(void)
-- 
2.53.0-Meta