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authorShakeel Butt <shakeel.butt@linux.dev>2026-05-25 20:39:31 -0700
committerAndrew Morton <akpm@linux-foundation.org>2026-05-28 21:32:03 -0700
commitdcc7a600136c8ace56a1278bd3f817100282680c (patch)
tree2b3d934547691ff63a753522c248ac77a27b6655 /mm
parent4c4a18bfa9a42505a7833413aa9c83f93fd0594b (diff)
downloadlinux-next-history-dcc7a600136c8ace56a1278bd3f817100282680c.tar.gz
memcg: multi objcg charge support
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 slot round-robin 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. Link: https://lore.kernel.org/20260526033931.1760588-5-shakeel.butt@linux.dev Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev> 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> Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org> Cc: Alexandre Ghiti <alex@ghiti.fr> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Joshua Hahn <joshua.hahnjy@gmail.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Muchun Song <muchun.song@linux.dev> Cc: Qi Zheng <qi.zheng@linux.dev> Cc: Roman Gushchin <roman.gushchin@linux.dev> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'mm')
-rw-r--r--mm/memcontrol.c200
1 files changed, 142 insertions, 58 deletions
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ac7c99e32f99a..e24114a4493a3 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 unsigned int as uint16_t cannot represent the full
- * sub-page remainder.
+e patches/memcg-uint16_t-for-nr_bytes-in-obj_stock_pcp.patch * 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)
generated by cgit 1.3-korg (git 2.53.0) at 2026-06-01 04:05:23 +0000