On 10/13/2026 4:54 AM, Max Kellermann wrote:
Rotating or onlining a memcg currently uses hlist_nulls_add_tail_rcu()
which walks the destination bin to find its tail while holding
`memcg_lru.lock` with interrupts disabled. With thousands of cgroups,
this takes a long time and causes severe lock contention.
Cache a pointer to each bin's last node, making tail insertion O(1).
This requires remembering the bin number (but the new struct field
occupies a hole, so it does not grow).
In a micro benchmark that creates 1500 cgroups and repeatedly writes
`1024M swappiness=0` to `memory.reclaim` in 8 threads, the request
rate increases by a factor of 9.5 and the average duration of each
write request is reduced by 89%.
All writes return EAGAIN in both runs; the benchmark measures the cost
of reclaim attempts that cannot meet their requested target, not
reclaimed throughput.
Signed-off-by: Max Kellermann <[email protected]>
---
include/linux/mmzone.h | 4 ++++
mm/vmscan.c | 24 +++++++++++++++++++++---
2 files changed, 25 insertions(+), 3 deletions(-)
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 94f9c3ff5416..9c1c1f153d1f 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -586,6 +586,8 @@ struct lru_gen_folio {
u8 gen;
/* the list segment this lru_gen_folio belongs to */
u8 seg;
+ /* the memcg bin, protected by memcg_lru.lock */
+ u8 bin;
/* per-node lru_gen_folio list for global reclaim */
struct hlist_nulls_node list;
};
@@ -683,6 +685,8 @@ struct lru_gen_memcg {
unsigned long nr_memcgs[MEMCG_NR_GENS];
/* per-node lru_gen_folio list for global reclaim */
struct hlist_nulls_head fifo[MEMCG_NR_GENS][MEMCG_NR_BINS];
+ /* last entry in each bin, or NULL if empty */
+ struct hlist_nulls_node *tail[MEMCG_NR_GENS][MEMCG_NR_BINS];
/* protects the above */
spinlock_t lock;
};
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 3d4bc12cf18e..69976df5d537 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4384,21 +4384,37 @@ static void lru_gen_add_memcg(struct lru_gen_memcg
*memcg_lru,
int gen, int bin, bool at_head)
{
struct hlist_nulls_head *head = &memcg_lru->fifo[gen][bin];
+ struct hlist_nulls_node *last = memcg_lru->tail[gen][bin];
lockdep_assert_held(&memcg_lru->lock);
- if (at_head)
+ lrugen->bin = bin;
+
+ if (at_head || !last)
hlist_nulls_add_head_rcu(&lrugen->list, head);
else
- hlist_nulls_add_tail_rcu(&lrugen->list, head);
+ hlist_nulls_add_behind_rcu(&lrugen->list, last);
+
+ if (!at_head || !last)
+ memcg_lru->tail[gen][bin] = &lrugen->list;
}
static void lru_gen_del_memcg(struct lru_gen_memcg *memcg_lru,
struct lru_gen_folio *lrugen,
int gen)
{
+ const int bin = lrugen->bin;
+ struct hlist_nulls_head *head = &memcg_lru->fifo[gen][bin];
+
lockdep_assert_held(&memcg_lru->lock);
+ /* pprev points to head->first for the only entry, otherwise prev->next. */
+ if (memcg_lru->tail[gen][bin] == &lrugen->list)
+ memcg_lru->tail[gen][bin] = lrugen->list.pprev == &head->first
+ ? NULL
+ : container_of(lrugen->list.pprev, struct
hlist_nulls_node, next);
+
+ /* Keep next intact for readers that still hold this entry. */
hlist_nulls_del_init_rcu(&lrugen->list);
}
@@ -5883,8 +5899,10 @@ void lru_gen_init_pgdat(struct pglist_data *pgdat)
spin_lock_init(&pgdat->memcg_lru.lock);
for (i = 0; i < MEMCG_NR_GENS; i++) {
- for (j = 0; j < MEMCG_NR_BINS; j++)
+ for (j = 0; j < MEMCG_NR_BINS; j++) {
INIT_HLIST_NULLS_HEAD(&pgdat->memcg_lru.fifo[i][j], i);
+ pgdat->memcg_lru.tail[i][j] = NULL;
+ }
}
}
Hi Max,
I've also submitted an RFC patch that addresses this issue.
https://lore.kernel.org/all/[email protected]/
--
Best regards
Ridong
