[PATCH 1/4] memcg: store node_id instead of pglist_data pointer

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

The struct obj_stock_pcp stores a pointer to pglist_data for the slab
stats cached on the cpu. On 64-bit machines, this costs 8 bytes. The
pointer is not strictly required: NODE_DATA() can recover it from the
node id. Replace cached_pgdat with int16_t node_id and use NUMA_NO_NODE
as the "no stats cached" sentinel.

At the moment all the archs limit MAX_NUMNODES to 1024 so int16_t is
plenty; a BUILD_BUG_ON() makes sure we notice if that ever changes.

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Tested-by: kernel test robot <oliver.sang@intel.com>
---
 mm/memcontrol.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b8caeb7ccaa3..d7c162946719 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2021,7 +2021,7 @@ struct obj_stock_pcp {
 	local_trylock_t lock;
 	unsigned int nr_bytes;
 	struct obj_cgroup *cached_objcg;
-	struct pglist_data *cached_pgdat;
+	int16_t node_id;
 	int nr_slab_reclaimable_b;
 	int nr_slab_unreclaimable_b;
 
@@ -2031,6 +2031,7 @@ struct obj_stock_pcp {
 
 static DEFINE_PER_CPU_ALIGNED(struct obj_stock_pcp, obj_stock) = {
 	.lock = INIT_LOCAL_TRYLOCK(lock),
+	.node_id = NUMA_NO_NODE,
 };
 
 static DEFINE_MUTEX(percpu_charge_mutex);
@@ -3159,6 +3160,13 @@ static void __account_obj_stock(struct obj_cgroup *objcg,
 {
 	int *bytes;
 
+	/*
+	 * Though at the moment MAX_NUMNODES <= 1024 in all archs but let's make
+	 * sure it does not exceed S16_MAX otherwise we need to fix node_id type
+	 * in struct obj_stock_pcp.
+	 */
+	BUILD_BUG_ON(MAX_NUMNODES >= S16_MAX);
+
 	if (!stock || READ_ONCE(stock->cached_objcg) != objcg)
 		goto direct;
 
@@ -3166,9 +3174,11 @@ static void __account_obj_stock(struct obj_cgroup *objcg,
 	 * Save vmstat data in stock and skip vmstat array update unless
 	 * accumulating over a page of vmstat data or when pgdat changes.
 	 */
-	if (stock->cached_pgdat != pgdat) {
+	if (stock->node_id == NUMA_NO_NODE) {
+		stock->node_id = pgdat->node_id;
+	} else if (stock->node_id != pgdat->node_id) {
 		/* Flush the existing cached vmstat data */
-		struct pglist_data *oldpg = stock->cached_pgdat;
+		struct pglist_data *oldpg = NODE_DATA(stock->node_id);
 
 		if (stock->nr_slab_reclaimable_b) {
 			mod_objcg_mlstate(objcg, oldpg, NR_SLAB_RECLAIMABLE_B,
@@ -3180,7 +3190,7 @@ static void __account_obj_stock(struct obj_cgroup *objcg,
 					  stock->nr_slab_unreclaimable_b);
 			stock->nr_slab_unreclaimable_b = 0;
 		}
-		stock->cached_pgdat = pgdat;
+		stock->node_id = pgdat->node_id;
 	}
 
 	bytes = (idx == NR_SLAB_RECLAIMABLE_B) ? &stock->nr_slab_reclaimable_b
@@ -3276,19 +3286,21 @@ static void drain_obj_stock(struct obj_stock_pcp *stock)
 	 * Flush the vmstat data in current stock
 	 */
 	if (stock->nr_slab_reclaimable_b || stock->nr_slab_unreclaimable_b) {
+		struct pglist_data *oldpg = NODE_DATA(stock->node_id);
+
 		if (stock->nr_slab_reclaimable_b) {
-			mod_objcg_mlstate(old, stock->cached_pgdat,
+			mod_objcg_mlstate(old, oldpg,
 					  NR_SLAB_RECLAIMABLE_B,
 					  stock->nr_slab_reclaimable_b);
 			stock->nr_slab_reclaimable_b = 0;
 		}
 		if (stock->nr_slab_unreclaimable_b) {
-			mod_objcg_mlstate(old, stock->cached_pgdat,
+			mod_objcg_mlstate(old, oldpg,
 					  NR_SLAB_UNRECLAIMABLE_B,
 					  stock->nr_slab_unreclaimable_b);
 			stock->nr_slab_unreclaimable_b = 0;
 		}
-		stock->cached_pgdat = NULL;
+		stock->node_id = NUMA_NO_NODE;
 	}
 
 	WRITE_ONCE(stock->cached_objcg, NULL);
-- 
2.53.0-Meta