[PATCH v3 3/4] memcg: int16_t for cached slab stats

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

Currently struct obj_stock_pcp stores cached slab stats in 'int' which
is 4 bytes per counter on 64-bit machines. Switch them to int16_t to
shrink the cached metadata.

The existing PAGE_SIZE flush in __account_obj_stock() bounds *bytes at
PAGE_SIZE on 4KiB and 16KiB page archs, well within int16_t. On 64KiB
pages PAGE_SIZE is well above S16_MAX so that flush never fires, and a
sufficiently long run of accumulations would overflow the cache. Add
an explicit S16_MAX guard before each add: when the next add would
push abs(*bytes) past S16_MAX, fold the cached value into @nr and
flush directly via mod_objcg_mlstate() before the accumulation.

Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type")
Tested-by: kernel test robot <oliver.sang@intel.com>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Acked-by: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
Changes since v2:
- Simplify code based on David Laight's suggestion.
- Collected tags

Changes since v1:
- Collected tags

 mm/memcontrol.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index be82e52c7999..fbe0e9915daa 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2035,8 +2035,8 @@ struct obj_stock_pcp {
 	struct obj_cgroup *cached_objcg;
 	obj_stock_bytes_t nr_bytes;
 	int16_t node_id;
-	int nr_slab_reclaimable_b;
-	int nr_slab_unreclaimable_b;
+	int16_t nr_slab_reclaimable_b;
+	int16_t nr_slab_unreclaimable_b;
 
 	struct work_struct work;
 	unsigned long flags;
@@ -3173,7 +3173,7 @@ static void __account_obj_stock(struct obj_cgroup *objcg,
 				struct obj_stock_pcp *stock, int nr,
 				struct pglist_data *pgdat, enum node_stat_item idx)
 {
-	int *bytes;
+	int16_t *bytes;
 
 	/*
 	 * Though at the moment MAX_NUMNODES <= 1024 in all archs but let's make
@@ -3210,21 +3210,20 @@ static void __account_obj_stock(struct obj_cgroup *objcg,
 
 	bytes = (idx == NR_SLAB_RECLAIMABLE_B) ? &stock->nr_slab_reclaimable_b
 					       : &stock->nr_slab_unreclaimable_b;
+
 	/*
-	 * Even for large object >= PAGE_SIZE, the vmstat data will still be
-	 * cached locally at least once before pushing it out.
+	 * Fold @nr into the cached value and decide whether to keep it cached
+	 * or flush it directly. Cache the combined value when it fits in the
+	 * int16_t storage and either the cache was empty (so even a value
+	 * above PAGE_SIZE gets a chance to be canceled by a paired delta) or
+	 * the combined value is within the PAGE_SIZE flush threshold.
 	 */
-	if (!*bytes) {
+	nr += *bytes;
+	if (abs(nr) <= S16_MAX && (!*bytes || abs(nr) <= PAGE_SIZE)) {
 		*bytes = nr;
 		nr = 0;
 	} else {
-		*bytes += nr;
-		if (abs(*bytes) > PAGE_SIZE) {
-			nr = *bytes;
-			*bytes = 0;
-		} else {
-			nr = 0;
-		}
+		*bytes = 0;
 	}
 direct:
 	if (nr)
-- 
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