[PATCH v3 0/4] memcg: shrink obj_stock_pcp and cache multiple objcgs

[PATCH v3 0/4] memcg: shrink obj_stock_pcp and cache multiple objcgs

[Shakeel Butt]

Commit 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg
per-node type") split a memcg's single obj_cgroup into one per NUMA
node so that reparenting LRU folios can take per-node lru locks. As a
side effect, the per-CPU obj_stock_pcp -- which caches a single
cached_objcg pointer -- thrashes on workloads where threads of the
same memcg run on different NUMA nodes. The kernel test robot reported
a 67.7% regression on stress-ng.switch.ops_per_sec from this pattern.

Commit d0211878ce06 ("memcg: cache obj_stock by memcg, not by objcg
pointer") landed as a temporary fix by treating sibling per-node
objcgs as equivalent for the cache lookup, intended to be reverted
once per-node kmem accounting is introduced. This series takes a more
general approach: cache multiple objcgs per CPU using the multi-slot
pattern memcg_stock_pcp already uses, so the per-node objcg variants
of one memcg can all coexist in the stock without ever forcing a
drain. The temporary fix can then be reverted.

To avoid increasing the per-CPU cache footprint, the first three
patches shrink the existing single-slot obj_stock_pcp fields.
The final patch converts cached_objcg and nr_bytes into
NR_OBJ_STOCK=5 slot arrays and reorders the struct so the entire
consume/refill/account hot path fits within a single 64-byte cache
line on non-debug 64-bit builds (verified with pahole).

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202605121641.b6a60cb0-lkp@intel.com
Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type")
Tested-by: kernel test robot <oliver.sang@intel.com>

Shakeel Butt (4):
  memcg: store node_id instead of pglist_data pointer
  memcg: uint16_t for nr_bytes in obj_stock_pcp
  memcg: int16_t for cached slab stats
  memcg: multi objcg charge support

 mm/memcontrol.c | 214 +++++++++++++++++++++++++++++++++++-------------
 1 file changed, 157 insertions(+), 57 deletions(-)

--

Changes since v2:
http://lore.kernel.org/20260522011908.1669332-1-shakeel.butt@linux.dev

- Fix comments (Muchun & Qi)
- Simplify code (David Laight)
- Fix handling of archs with base page size larger than 256 KiB (Sashiko)

Changes since v1:
http://lore.kernel.org/20260520053123.2709959-1-shakeel.butt@linux.dev

- Collected review tags (Harry & Muchun)
- Fix comparison operators (Harry)
- Use round robin for drain

2.53.0-Meta

[PATCH v3 1/4] memcg: store node_id instead of pglist_data pointer

[Shakeel Butt]

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.

Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type")
Tested-by: kernel test robot <oliver.sang@intel.com>
Acked-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
Changes since v1 & v2:
- Added tags in the commit message

 mm/memcontrol.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 13f5d4b2a78e..9bee9031171f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2022,7 +2022,7 @@ 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;
 
@@ -2032,6 +2032,7 @@ 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);
@@ -3162,6 +3163,13 @@ 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;
 
@@ -3169,9 +3177,11 @@ 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,
@@ -3183,7 +3193,7 @@ 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
@@ -3279,19 +3289,21 @@ 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

[PATCH v3 2/4] memcg: uint16_t for nr_bytes in obj_stock_pcp

[Shakeel Butt]

Currently struct obj_stock_pcp stores nr_bytes in an 'unsigned int'
which is 4 bytes on 64-bit machines. Switch the field to uint16_t to
shrink the per-CPU cache.

The kernel supports PAGE_SIZE_4KB, _8KB, _16KB, _32KB, _64KB and
_256KB (see HAVE_PAGE_SIZE_* in arch/Kconfig). After the
PAGE_SIZE-aligned flush in __refill_obj_stock(), the sub-page
remainder fits in uint16_t up through 64KiB pages where PAGE_SIZE - 1
== U16_MAX, but on 256KiB pages PAGE_SIZE - 1 == 0x3FFFF exceeds
U16_MAX. The accumulator also needs to stay within uint16_t between
page-aligned flushes on 64KiB pages where PAGE_SIZE itself is
U16_MAX + 1.

Accumulate the new total in an 'unsigned int' local, then on
PAGE_SHIFT <= 16 flush whenever the accumulator would hit U16_MAX;
together with the existing allow_uncharge flush at PAGE_SIZE this
keeps the uint16_t safe.

On configs with PAGE_SHIFT > 16 (PAGE_SIZE_256KB on hexagon and
powerpc 44x, both 32-bit), uint16_t cannot represent the sub-page
remainder. Define obj_stock_bytes_t as 'unsigned int' on those
archs so nr_bytes can hold the full remainder and the normal
page-boundary flush in __refill_obj_stock() and the page extraction
in drain_obj_stock() both work correctly.

The single-cache-line layout target only applies to PAGE_SHIFT <= 16;
those archs are 32-bit embedded and not the optimization target.

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:
- Based on Sashiko's concern regarding archs with 256KiB base pages, added
  special handling for such arch and simplify the code overall.
- Updated commit message.
- Have kept the review tags as overall code remains same mostly. 

Changes since v1:
- Collected tags
- Rearrange fields of obj_stock_pcp (David Laight)
- Fix comparison operator (Harry)

 mm/memcontrol.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9bee9031171f..8c1b65e6da5d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2020,8 +2020,17 @@ static DEFINE_PER_CPU_ALIGNED(struct memcg_stock_pcp, memcg_stock) = {
 
 struct obj_stock_pcp {
 	local_trylock_t lock;
-	unsigned int nr_bytes;
 	struct obj_cgroup *cached_objcg;
+#if PAGE_SHIFT > 16
+	/*
+	 * On rare archs with 256KiB base page size (hexagon and powerpc 44x)
+	 * keep nr_bytes to unsigned int as uint16_t cannot represent the full
+	 * sub-page remainder.
+	 */
+	unsigned int nr_bytes;
+#else
+	uint16_t nr_bytes;
+#endif
 	int16_t node_id;
 	int nr_slab_reclaimable_b;
 	int nr_slab_unreclaimable_b;
@@ -3334,6 +3343,7 @@ static void __refill_obj_stock(struct obj_cgroup *objcg,
 			       bool allow_uncharge)
 {
 	unsigned int nr_pages = 0;
+	unsigned int stock_nr_bytes;
 
 	if (!stock) {
 		nr_pages = nr_bytes >> PAGE_SHIFT;
@@ -3342,21 +3352,24 @@ static void __refill_obj_stock(struct obj_cgroup *objcg,
 		goto out;
 	}
 
+	stock_nr_bytes = stock->nr_bytes;
 	if (READ_ONCE(stock->cached_objcg) != objcg) { /* reset if necessary */
 		drain_obj_stock(stock);
 		obj_cgroup_get(objcg);
-		stock->nr_bytes = atomic_read(&objcg->nr_charged_bytes)
+		stock_nr_bytes = atomic_read(&objcg->nr_charged_bytes)
 				? atomic_xchg(&objcg->nr_charged_bytes, 0) : 0;
 		WRITE_ONCE(stock->cached_objcg, objcg);
 
 		allow_uncharge = true;	/* Allow uncharge when objcg changes */
 	}
-	stock->nr_bytes += nr_bytes;
+	stock_nr_bytes += nr_bytes;
 
-	if (allow_uncharge && (stock->nr_bytes > PAGE_SIZE)) {
-		nr_pages = stock->nr_bytes >> PAGE_SHIFT;
-		stock->nr_bytes &= (PAGE_SIZE - 1);
+	if ((allow_uncharge && (stock_nr_bytes > PAGE_SIZE)) ||
+	    stock_nr_bytes > U16_MAX) {
+		nr_pages = stock_nr_bytes >> PAGE_SHIFT;
+		stock_nr_bytes &= (PAGE_SIZE - 1);
 	}
+	stock->nr_bytes = stock_nr_bytes;
 
 out:
 	if (nr_pages)
-- 
2.53.0-Meta

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

[Shakeel Butt]

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)
-- 
2.53.0-Meta

[PATCH v3 4/4] memcg: multi objcg charge support

[Shakeel Butt]

Commit 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg
per-node type") split a memcg's single obj_cgroup into one per NUMA
node so that reparenting LRU folios can take per-node lru locks. As a
side effect, the per-CPU obj_stock_pcp -- which caches exactly one
cached_objcg -- thrashes on workloads where threads of the same memcg
run on different NUMA nodes. The kernel test robot reported a 67.7%
regression on stress-ng.switch.ops_per_sec from this pattern.

Mirror the multi-slot pattern already used by memcg_stock_pcp: turn
nr_bytes and cached_objcg into NR_OBJ_STOCK-element arrays, scan all
slots on consume/refill/account, prefer empty slots when inserting,
and evict a random slot only when full. With multiple slots a CPU can
hold the per-node objcg variants of one memcg plus a few siblings
without ever forcing a drain.

A single int8_t index records which slot the cached slab stats belong
to; the stats are flushed on slot or pgdat change. With NR_OBJ_STOCK
= 5 the layout (verified with pahole) is:

  offset 0  : lock(1) + index(1) + node_id(2) + slab stats(4) = 8B
  offset 8  : nr_bytes[5]                                     = 10B
  offset 18 : padding                                         = 6B
  offset 24 : cached[5]                                       = 40B
  offset 64 : (line 2) work_struct + flags (cold)

so consume_obj_stock, refill_obj_stock and the slab account path each
touch exactly one 64-byte cache line on non-debug 64-bit builds.

Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202605121641.b6a60cb0-lkp@intel.com
Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type")
Tested-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
Changes since v2:
- fix comments (Muchun)

Changes since v1:
- Use round robin for drain

 mm/memcontrol.c | 200 ++++++++++++++++++++++++++++++++++--------------
 1 file changed, 142 insertions(+), 58 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 24b6519eed9d..d5687e0797a0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -150,15 +150,15 @@ static void obj_cgroup_release(struct percpu_ref *ref)
 	 * However, it can be PAGE_SIZE or (x * PAGE_SIZE).
 	 *
 	 * The following sequence can lead to it:
-	 * 1) CPU0: objcg == stock->cached_objcg
+	 * 1) CPU0: objcg cached in one of stock->cached[i]
 	 * 2) CPU1: we do a small allocation (e.g. 92 bytes),
 	 *          PAGE_SIZE bytes are charged
 	 * 3) CPU1: a process from another memcg is allocating something,
 	 *          the stock if flushed,
 	 *          objcg->nr_charged_bytes = PAGE_SIZE - 92
-	 * 5) CPU0: we do release this object,
-	 *          92 bytes are added to stock->nr_bytes
-	 * 6) CPU0: stock is flushed,
+	 * 4) CPU0: we do release this object,
+	 *          92 bytes are added to stock->nr_bytes[i]
+	 * 5) CPU0: stock is flushed,
 	 *          92 bytes are added to objcg->nr_charged_bytes
 	 *
 	 * In the result, nr_charged_bytes == PAGE_SIZE.
@@ -2018,34 +2018,49 @@ static DEFINE_PER_CPU_ALIGNED(struct memcg_stock_pcp, memcg_stock) = {
 	.lock = INIT_LOCAL_TRYLOCK(lock),
 };
 
+/*
+ * NR_OBJ_STOCK is sized so the entire hot path of obj_stock_pcp
+ * (lock, accounting metadata, nr_bytes[] and cached[]) fits within a
+ * single 64-byte cache line on non-debug 64-bit builds. With 5 slots:
+ *   lock(1) + index(1) + node_id(2) + slab stats(4) + nr_bytes(10)
+ *   + pad(6) + cached(40) == 64 bytes.
+ * A CPU can thus consume/refill/account against five different objcgs
+ * (typically per-node variants of the same memcg) while incurring at
+ * most one cache miss on the stock.
+ */
+#define NR_OBJ_STOCK 5
 struct obj_stock_pcp {
 	local_trylock_t lock;
-	struct obj_cgroup *cached_objcg;
+	int8_t index;
+	int16_t node_id;
+	int16_t nr_slab_reclaimable_b;
+	int16_t nr_slab_unreclaimable_b;
 #if PAGE_SHIFT > 16
 	/*
 	 * On rare archs with 256KiB base page size (hexagon and powerpc 44x)
 	 * keep nr_bytes to unsinged int as uint16_t cannot represent the full
-	 * sub-page remainder.
+	 * sub-page remainder. Such archs are not cacheline optimization target.
 	 */
-	unsigned int nr_bytes;
+	unsigned int nr_bytes[NR_OBJ_STOCK];
 #else
-	uint16_t nr_bytes;
+	uint16_t nr_bytes[NR_OBJ_STOCK];
 #endif
-	int16_t node_id;
-	int16_t nr_slab_reclaimable_b;
-	int16_t nr_slab_unreclaimable_b;
+	struct obj_cgroup *cached[NR_OBJ_STOCK];
 
 	struct work_struct work;
 	unsigned long flags;
+	uint8_t drain_idx;
 };
 
 static DEFINE_PER_CPU_ALIGNED(struct obj_stock_pcp, obj_stock) = {
 	.lock = INIT_LOCAL_TRYLOCK(lock),
+	.index = -1,
 	.node_id = NUMA_NO_NODE,
 };
 
 static DEFINE_MUTEX(percpu_charge_mutex);
 
+static void drain_obj_stock_slot(struct obj_stock_pcp *stock, int i);
 static void drain_obj_stock(struct obj_stock_pcp *stock);
 static bool obj_stock_flush_required(struct obj_stock_pcp *stock,
 				     struct mem_cgroup *root_memcg);
@@ -3165,12 +3180,13 @@ static void unlock_stock(struct obj_stock_pcp *stock)
 		local_unlock(&obj_stock.lock);
 }
 
-/* Call after __refill_obj_stock() to ensure stock->cached_objg == objcg */
+/* Call after __refill_obj_stock() so a slot for objcg exists in the stock */
 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)
 {
 	int16_t *bytes;
+	int i;
 
 	/*
 	 * Though at the moment MAX_NUMNODES <= 1024 in all archs but let's make
@@ -3179,29 +3195,39 @@ static void __account_obj_stock(struct obj_cgroup *objcg,
 	 */
 	BUILD_BUG_ON(MAX_NUMNODES >= S16_MAX);
 
-	if (!stock || READ_ONCE(stock->cached_objcg) != objcg)
+	if (!stock)
+		goto direct;
+
+	for (i = 0; i < NR_OBJ_STOCK; ++i) {
+		if (READ_ONCE(stock->cached[i]) == objcg)
+			break;
+	}
+	if (i == NR_OBJ_STOCK)
 		goto direct;
 
 	/*
 	 * Save vmstat data in stock and skip vmstat array update unless
-	 * accumulating over a page of vmstat data or when pgdat changes.
+	 * accumulating over a page of vmstat data or when the objcg slot or
+	 * pgdat the stats belong to changes.
 	 */
-	if (stock->node_id == NUMA_NO_NODE) {
+	if (stock->index < 0) {
+		stock->index = i;
 		stock->node_id = pgdat->node_id;
-	} else if (stock->node_id != pgdat->node_id) {
-		/* Flush the existing cached vmstat data */
+	} else if (stock->index != i || stock->node_id != pgdat->node_id) {
+		struct obj_cgroup *old = READ_ONCE(stock->cached[stock->index]);
 		struct pglist_data *oldpg = NODE_DATA(stock->node_id);
 
 		if (stock->nr_slab_reclaimable_b) {
-			mod_objcg_mlstate(objcg, oldpg, NR_SLAB_RECLAIMABLE_B,
+			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(objcg, oldpg, NR_SLAB_UNRECLAIMABLE_B,
+			mod_objcg_mlstate(old, oldpg, NR_SLAB_UNRECLAIMABLE_B,
 					  stock->nr_slab_unreclaimable_b);
 			stock->nr_slab_unreclaimable_b = 0;
 		}
+		stock->index = i;
 		stock->node_id = pgdat->node_id;
 	}
 
@@ -3231,10 +3257,16 @@ static bool __consume_obj_stock(struct obj_cgroup *objcg,
 				struct obj_stock_pcp *stock,
 				unsigned int nr_bytes)
 {
-	if (objcg == READ_ONCE(stock->cached_objcg) &&
-	    stock->nr_bytes >= nr_bytes) {
-		stock->nr_bytes -= nr_bytes;
-		return true;
+	int i;
+
+	for (i = 0; i < NR_OBJ_STOCK; ++i) {
+		if (READ_ONCE(stock->cached[i]) != objcg)
+			continue;
+		if (stock->nr_bytes[i] >= nr_bytes) {
+			stock->nr_bytes[i] -= nr_bytes;
+			return true;
+		}
+		return false;
 	}
 
 	return false;
@@ -3255,16 +3287,42 @@ static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
 	return ret;
 }
 
-static void drain_obj_stock(struct obj_stock_pcp *stock)
+/* Flush the cached slab stats (if any) back to their owning objcg/pgdat. */
+static void drain_obj_stock_stats(struct obj_stock_pcp *stock)
+{
+	struct obj_cgroup *old;
+	struct pglist_data *oldpg;
+
+	if (stock->index < 0)
+		return;
+
+	old = READ_ONCE(stock->cached[stock->index]);
+	oldpg = NODE_DATA(stock->node_id);
+
+	if (stock->nr_slab_reclaimable_b) {
+		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, oldpg, NR_SLAB_UNRECLAIMABLE_B,
+				  stock->nr_slab_unreclaimable_b);
+		stock->nr_slab_unreclaimable_b = 0;
+	}
+	stock->index = -1;
+	stock->node_id = NUMA_NO_NODE;
+}
+
+static void drain_obj_stock_slot(struct obj_stock_pcp *stock, int i)
 {
-	struct obj_cgroup *old = READ_ONCE(stock->cached_objcg);
+	struct obj_cgroup *old = READ_ONCE(stock->cached[i]);
 
 	if (!old)
 		return;
 
-	if (stock->nr_bytes) {
-		unsigned int nr_pages = stock->nr_bytes >> PAGE_SHIFT;
-		unsigned int nr_bytes = stock->nr_bytes & (PAGE_SIZE - 1);
+	if (stock->nr_bytes[i]) {
+		unsigned int nr_pages = stock->nr_bytes[i] >> PAGE_SHIFT;
+		unsigned int nr_bytes = stock->nr_bytes[i] & (PAGE_SIZE - 1);
 
 		if (nr_pages) {
 			struct mem_cgroup *memcg;
@@ -3290,46 +3348,43 @@ static void drain_obj_stock(struct obj_stock_pcp *stock)
 		 * so it might be changed in the future.
 		 */
 		atomic_add(nr_bytes, &old->nr_charged_bytes);
-		stock->nr_bytes = 0;
+		stock->nr_bytes[i] = 0;
 	}
 
-	/*
-	 * 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);
+	/* Flush vmstat data when its owning slot is being drained. */
+	if (stock->index == i)
+		drain_obj_stock_stats(stock);
 
-		if (stock->nr_slab_reclaimable_b) {
-			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, oldpg,
-					  NR_SLAB_UNRECLAIMABLE_B,
-					  stock->nr_slab_unreclaimable_b);
-			stock->nr_slab_unreclaimable_b = 0;
-		}
-		stock->node_id = NUMA_NO_NODE;
-	}
-
-	WRITE_ONCE(stock->cached_objcg, NULL);
+	WRITE_ONCE(stock->cached[i], NULL);
 	obj_cgroup_put(old);
 }
 
+static void drain_obj_stock(struct obj_stock_pcp *stock)
+{
+	int i;
+
+	for (i = 0; i < NR_OBJ_STOCK; ++i)
+		drain_obj_stock_slot(stock, i);
+}
+
 static bool obj_stock_flush_required(struct obj_stock_pcp *stock,
 				     struct mem_cgroup *root_memcg)
 {
-	struct obj_cgroup *objcg = READ_ONCE(stock->cached_objcg);
+	struct obj_cgroup *objcg;
 	struct mem_cgroup *memcg;
 	bool flush = false;
+	int i;
 
 	rcu_read_lock();
-	if (objcg) {
+	for (i = 0; i < NR_OBJ_STOCK; ++i) {
+		objcg = READ_ONCE(stock->cached[i]);
+		if (!objcg)
+			continue;
 		memcg = obj_cgroup_memcg(objcg);
-		if (memcg && mem_cgroup_is_descendant(memcg, root_memcg))
+		if (memcg && mem_cgroup_is_descendant(memcg, root_memcg)) {
 			flush = true;
+			break;
+		}
 	}
 	rcu_read_unlock();
 
@@ -3343,6 +3398,7 @@ static void __refill_obj_stock(struct obj_cgroup *objcg,
 {
 	unsigned int nr_pages = 0;
 	unsigned int stock_nr_bytes;
+	int i, slot = -1, empty_slot = -1;
 
 	if (!stock) {
 		nr_pages = nr_bytes >> PAGE_SHIFT;
@@ -3351,16 +3407,44 @@ static void __refill_obj_stock(struct obj_cgroup *objcg,
 		goto out;
 	}
 
-	stock_nr_bytes = stock->nr_bytes;
-	if (READ_ONCE(stock->cached_objcg) != objcg) { /* reset if necessary */
-		drain_obj_stock(stock);
+	for (i = 0; i < NR_OBJ_STOCK; ++i) {
+		struct obj_cgroup *cached = READ_ONCE(stock->cached[i]);
+
+		if (!cached) {
+			if (empty_slot == -1)
+				empty_slot = i;
+			continue;
+		}
+		if (cached == objcg) {
+			slot = i;
+			break;
+		}
+	}
+
+	if (slot == -1) {
+		slot = empty_slot;
+		if (slot == -1) {
+			slot = stock->drain_idx++;
+			if (stock->drain_idx == NR_OBJ_STOCK)
+				stock->drain_idx = 0;
+			drain_obj_stock_slot(stock, slot);
+		}
 		obj_cgroup_get(objcg);
+		/*
+		 * Keep the xchg result in the unsigned int local; storing
+		 * it directly into stock->nr_bytes[slot] (uint16_t) would
+		 * silently truncate values >= U16_MAX and bypass the flush
+		 * guard below, leaking page-counter charges.
+		 */
 		stock_nr_bytes = atomic_read(&objcg->nr_charged_bytes)
 				? atomic_xchg(&objcg->nr_charged_bytes, 0) : 0;
-		WRITE_ONCE(stock->cached_objcg, objcg);
+		WRITE_ONCE(stock->cached[slot], objcg);
 
 		allow_uncharge = true;	/* Allow uncharge when objcg changes */
+	} else {
+		stock_nr_bytes = stock->nr_bytes[slot];
 	}
+
 	stock_nr_bytes += nr_bytes;
 
 	if ((allow_uncharge && (stock_nr_bytes > PAGE_SIZE)) ||
@@ -3368,7 +3452,7 @@ static void __refill_obj_stock(struct obj_cgroup *objcg,
 		nr_pages = stock_nr_bytes >> PAGE_SHIFT;
 		stock_nr_bytes &= (PAGE_SIZE - 1);
 	}
-	stock->nr_bytes = stock_nr_bytes;
+	stock->nr_bytes[slot] = stock_nr_bytes;
 
 out:
 	if (nr_pages)
-- 
2.53.0-Meta

Re: [PATCH v3 4/4] memcg: multi objcg charge support

[Harry Yoo]

On 5/26/26 12:39 PM, Shakeel Butt wrote:
> Commit 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg
> per-node type") split a memcg's single obj_cgroup into one per NUMA
> node so that reparenting LRU folios can take per-node lru locks. As a
> side effect, the per-CPU obj_stock_pcp -- which caches exactly one
> cached_objcg -- thrashes on workloads where threads of the same memcg
> run on different NUMA nodes. The kernel test robot reported a 67.7%
> regression on stress-ng.switch.ops_per_sec from this pattern.
> 
> Mirror the multi-slot pattern already used by memcg_stock_pcp: turn
> nr_bytes and cached_objcg into NR_OBJ_STOCK-element arrays, scan all
> slots on consume/refill/account, prefer empty slots when inserting,
> and evict a random slot only when full. With multiple slots a CPU can
> hold the per-node objcg variants of one memcg plus a few siblings
> without ever forcing a drain.

nit: evict a random slot -> evict a slot round-robin
> A single int8_t index records which slot the cached slab stats belong
> to; the stats are flushed on slot or pgdat change. With NR_OBJ_STOCK
> = 5 the layout (verified with pahole) is:
> 
>    offset 0  : lock(1) + index(1) + node_id(2) + slab stats(4) = 8B
>    offset 8  : nr_bytes[5]                                     = 10B
>    offset 18 : padding                                         = 6B
>    offset 24 : cached[5]                                       = 40B
>    offset 64 : (line 2) work_struct + flags (cold)
> 
> so consume_obj_stock, refill_obj_stock and the slab account path each
> touch exactly one 64-byte cache line on non-debug 64-bit builds.
> 
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202605121641.b6a60cb0-lkp@intel.com
> Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type")
> Tested-by: kernel test robot <oliver.sang@intel.com>
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> ---

Looks good to me,
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>

-- 
Cheers,
Harry / Hyeonggon