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memcg: multi objcg charge support

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Commit 01b9da291c ("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: 01b9da291c ("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>
This commit is contained in:
Shakeel Butt 2026-05-25 20:39:31 -07:00 committed by Andrew Morton
parent 7a09fb91c2
commit 29a1ea4145
1 changed files with 142 additions and 58 deletions
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200
mm/memcontrol.c
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@ -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 = READ_ONCE(stock->cached_objcg);
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[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)