[PATCH v3 1/8] memcg: reduce memory size of mem_cgroup_events_index

[Shakeel Butt <shakeel.butt@linux.dev>]

mem_cgroup_events_index is a translation table to get the right index of
the memcg relevant entry for the general vm_event_item. At the moment,
it is defined as integer array. However on a typical system the max
entry of vm_event_item (NR_VM_EVENT_ITEMS) is 113, so we don't need to
use int as storage type of the array. For now just use int8_t as type
and add a BUILD_BUG_ON() and will switch to short once NR_VM_EVENT_ITEMS
touches 127.

Another benefit of this change is that the translation table fits in 2
cachelines while previously it would require 8 cachelines (assuming 64
bytes cachesline).

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
---
Changes since v2:
- Used S8_MAX instead of 127
- Update commit message based on Yosry's feedback.

 mm/memcontrol.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 602ad5faad4d..c146187cda9c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -607,11 +607,13 @@ static const unsigned int memcg_vm_event_stat[] = {
 };
 
 #define NR_MEMCG_EVENTS ARRAY_SIZE(memcg_vm_event_stat)
-static int mem_cgroup_events_index[NR_VM_EVENT_ITEMS] __read_mostly;
+static int8_t mem_cgroup_events_index[NR_VM_EVENT_ITEMS] __read_mostly;
 
 static void init_memcg_events(void)
 {
-	int i;
+	int8_t i;
+
+	BUILD_BUG_ON(NR_VM_EVENT_ITEMS >= S8_MAX);
 
 	for (i = 0; i < NR_MEMCG_EVENTS; ++i)
 		mem_cgroup_events_index[memcg_vm_event_stat[i]] = i + 1;
-- 
2.43.0