[RFC PATCH -next 0/2] mm/mglru: remove memcg lru

[RFC PATCH -next 0/2] mm/mglru: remove memcg lru

[Chen Ridong]

From: Chen Ridong <chenridong@huawei.com>

The memcg LRU was introduced for global reclaim to improve scalability,
but its implementation has grown complex. Moreover, it can cause
performance regressions when creating a large number of memory cgroups [1].

This series implements mem_cgroup_iter with a reclaim cookie in
shrink_many() for global reclaim, following the pattern already established
in shrink_node_memcgs(), an approach suggested by Johannes [1]. The new
approach provides good fairness across cgroups by preserving iteration
state between reclaim passes.

Testing was performed using the original stress test from Zhao Yu [2] on a
1 TB, 4-node NUMA system. The results show:

                                            before         after
    stddev(pgsteal) / mean(pgsteal)            91.2%         75.7%
    sum(pgsteal) / sum(requested)             216.4%        230.5%

The new implementation reduces the standard deviation relative to the mean
by 15.5 percentage points, indicating improved fairness in memory reclaim
distribution. The total pages reclaimed increased from 85,086,871 to
90,633,890 (6.5% increase), resulting in a higher ratio of actual to
requested reclaim.

To simplify review:
- Patch 1 uses mem_cgroup_iter with reclaim cookie in shrink_many()
- Patch 2 removes the now-unused memcg LRU code

[1] https://lore.kernel.org/r/20251126171513.GC135004@cmpxchg.org
[2] https://lore.kernel.org/r/20221222041905.2431096-7-yuzhao@google.com

Chen Ridong (2):
  mm/mglru: use mem_cgroup_iter for global reclaim
  mm/mglru: remove memcg lru

 Documentation/mm/multigen_lru.rst |  30 ----
 include/linux/mmzone.h            |  89 ----------
 mm/memcontrol-v1.c                |   6 -
 mm/memcontrol.c                   |   4 -
 mm/mm_init.c                      |   1 -
 mm/vmscan.c                       | 270 ++++--------------------------
 6 files changed, 37 insertions(+), 363 deletions(-)

-- 
2.34.1

[RFC PATCH -next 1/2] mm/mglru: use mem_cgroup_iter for global reclaim

[Chen Ridong]

From: Chen Ridong <chenridong@huawei.com>

The memcg LRU was originally introduced for global reclaim to enhance
scalability. However, its implementation complexity has led to performance
regressions when dealing with a large number of memory cgroups [1].

As suggested by Johannes [1], this patch adopts mem_cgroup_iter with
cookie-based iteration for global reclaim, aligning with the approach
already used in shrink_node_memcgs. This simplification removes the
dedicated memcg LRU tracking while maintaining the core functionality.

It performed a stress test based on Zhao Yu's methodology [2] on a
1 TB, 4-node NUMA system. The results are summarized below:

					memcg LRU    memcg iter
stddev(pgsteal) / mean(pgsteal)            91.2%         75.7%
sum(pgsteal) / sum(requested)             216.4%        230.5%

The new implementation demonstrates a significant improvement in
fairness, reducing the standard deviation relative to the mean by
15.5 percentage points. While the reclaim accuracy shows a slight
increase in overscan (from 85086871 to 90633890, 6.5%).

The primary benefits of this change are:
1. Simplified codebase by removing custom memcg LRU infrastructure
2. Improved fairness in memory reclaim across multiple cgroups
3. Better performance when creating many memory cgroups

[1] https://lore.kernel.org/r/20251126171513.GC135004@cmpxchg.org
[2] https://lore.kernel.org/r/20221222041905.2431096-7-yuzhao@google.com
Signed-off-by: Chen Ridong <chenridong@huawei.com>
---
 mm/vmscan.c | 117 ++++++++++++++++------------------------------------
 1 file changed, 36 insertions(+), 81 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index fddd168a9737..70b0e7e5393c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4895,27 +4895,14 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 	return nr_to_scan < 0;
 }
 
-static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
+static void shrink_one(struct lruvec *lruvec, struct scan_control *sc)
 {
-	bool success;
 	unsigned long scanned = sc->nr_scanned;
 	unsigned long reclaimed = sc->nr_reclaimed;
-	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
 	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
 
-	/* lru_gen_age_node() called mem_cgroup_calculate_protection() */
-	if (mem_cgroup_below_min(NULL, memcg))
-		return MEMCG_LRU_YOUNG;
-
-	if (mem_cgroup_below_low(NULL, memcg)) {
-		/* see the comment on MEMCG_NR_GENS */
-		if (READ_ONCE(lruvec->lrugen.seg) != MEMCG_LRU_TAIL)
-			return MEMCG_LRU_TAIL;
-
-		memcg_memory_event(memcg, MEMCG_LOW);
-	}
-
-	success = try_to_shrink_lruvec(lruvec, sc);
+	try_to_shrink_lruvec(lruvec, sc);
 
 	shrink_slab(sc->gfp_mask, pgdat->node_id, memcg, sc->priority);
 
@@ -4924,86 +4911,55 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
 			   sc->nr_reclaimed - reclaimed);
 
 	flush_reclaim_state(sc);
-
-	if (success && mem_cgroup_online(memcg))
-		return MEMCG_LRU_YOUNG;
-
-	if (!success && lruvec_is_sizable(lruvec, sc))
-		return 0;
-
-	/* one retry if offlined or too small */
-	return READ_ONCE(lruvec->lrugen.seg) != MEMCG_LRU_TAIL ?
-	       MEMCG_LRU_TAIL : MEMCG_LRU_YOUNG;
 }
 
 static void shrink_many(struct pglist_data *pgdat, struct scan_control *sc)
 {
-	int op;
-	int gen;
-	int bin;
-	int first_bin;
-	struct lruvec *lruvec;
-	struct lru_gen_folio *lrugen;
+	struct mem_cgroup *target = sc->target_mem_cgroup;
+	struct mem_cgroup_reclaim_cookie reclaim = {
+		.pgdat = pgdat,
+	};
+	struct mem_cgroup_reclaim_cookie *cookie = &reclaim;
 	struct mem_cgroup *memcg;
-	struct hlist_nulls_node *pos;
 
-	gen = get_memcg_gen(READ_ONCE(pgdat->memcg_lru.seq));
-	bin = first_bin = get_random_u32_below(MEMCG_NR_BINS);
-restart:
-	op = 0;
-	memcg = NULL;
-
-	rcu_read_lock();
+	if (current_is_kswapd() || sc->memcg_full_walk)
+		cookie = NULL;
 
-	hlist_nulls_for_each_entry_rcu(lrugen, pos, &pgdat->memcg_lru.fifo[gen][bin], list) {
-		if (op) {
-			lru_gen_rotate_memcg(lruvec, op);
-			op = 0;
-		}
+	memcg = mem_cgroup_iter(target, NULL, cookie);
+	while (memcg) {
+		struct lruvec *lruvec = mem_cgroup_lruvec(memcg, pgdat);
 
-		mem_cgroup_put(memcg);
-		memcg = NULL;
+		cond_resched();
 
-		if (gen != READ_ONCE(lrugen->gen))
-			continue;
+		mem_cgroup_calculate_protection(target, memcg);
 
-		lruvec = container_of(lrugen, struct lruvec, lrugen);
-		memcg = lruvec_memcg(lruvec);
+		if (mem_cgroup_below_min(target, memcg))
+			goto next;
 
-		if (!mem_cgroup_tryget(memcg)) {
-			lru_gen_release_memcg(memcg);
-			memcg = NULL;
-			continue;
+		if (mem_cgroup_below_low(target, memcg)) {
+			if (!sc->memcg_low_reclaim) {
+				sc->memcg_low_skipped = 1;
+				goto next;
+			}
+			memcg_memory_event(memcg, MEMCG_LOW);
 		}
 
-		rcu_read_unlock();
+		shrink_one(lruvec, sc);
 
-		op = shrink_one(lruvec, sc);
-
-		rcu_read_lock();
-
-		if (should_abort_scan(lruvec, sc))
+		if (should_abort_scan(lruvec, sc)) {
+			if (cookie)
+				mem_cgroup_iter_break(target, memcg);
 			break;
-	}
-
-	rcu_read_unlock();
-
-	if (op)
-		lru_gen_rotate_memcg(lruvec, op);
-
-	mem_cgroup_put(memcg);
-
-	if (!is_a_nulls(pos))
-		return;
+		}
 
-	/* restart if raced with lru_gen_rotate_memcg() */
-	if (gen != get_nulls_value(pos))
-		goto restart;
+next:
+		if (cookie && sc->nr_reclaimed >= sc->nr_to_reclaim) {
+			mem_cgroup_iter_break(target, memcg);
+			break;
+		}
 
-	/* try the rest of the bins of the current generation */
-	bin = get_memcg_bin(bin + 1);
-	if (bin != first_bin)
-		goto restart;
+		memcg = mem_cgroup_iter(target, memcg, cookie);
+	}
 }
 
 static void lru_gen_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
@@ -5019,8 +4975,7 @@ static void lru_gen_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc
 
 	set_mm_walk(NULL, sc->proactive);
 
-	if (try_to_shrink_lruvec(lruvec, sc))
-		lru_gen_rotate_memcg(lruvec, MEMCG_LRU_YOUNG);
+	try_to_shrink_lruvec(lruvec, sc);
 
 	clear_mm_walk();
 
-- 
2.34.1

[RFC PATCH -next 2/2] mm/mglru: remove memcg lru

[Chen Ridong]

From: Chen Ridong <chenridong@huawei.com>

Now that the previous patch has switched global reclaim to use
mem_cgroup_iter, the specialized memcg LRU infrastructure is no longer
needed. This patch removes all related code:

Signed-off-by: Chen Ridong <chenridong@huawei.com>
---
 Documentation/mm/multigen_lru.rst |  30 ------
 include/linux/mmzone.h            |  89 -----------------
 mm/memcontrol-v1.c                |   6 --
 mm/memcontrol.c                   |   4 -
 mm/mm_init.c                      |   1 -
 mm/vmscan.c                       | 153 +-----------------------------
 6 files changed, 1 insertion(+), 282 deletions(-)

diff --git a/Documentation/mm/multigen_lru.rst b/Documentation/mm/multigen_lru.rst
index 52ed5092022f..bf8547e2f592 100644
--- a/Documentation/mm/multigen_lru.rst
+++ b/Documentation/mm/multigen_lru.rst
@@ -220,36 +220,6 @@ time domain because a CPU can scan pages at different rates under
 varying memory pressure. It calculates a moving average for each new
 generation to avoid being permanently locked in a suboptimal state.
 
-Memcg LRU
----------
-An memcg LRU is a per-node LRU of memcgs. It is also an LRU of LRUs,
-since each node and memcg combination has an LRU of folios (see
-``mem_cgroup_lruvec()``). Its goal is to improve the scalability of
-global reclaim, which is critical to system-wide memory overcommit in
-data centers. Note that memcg LRU only applies to global reclaim.
-
-The basic structure of an memcg LRU can be understood by an analogy to
-the active/inactive LRU (of folios):
-
-1. It has the young and the old (generations), i.e., the counterparts
-   to the active and the inactive;
-2. The increment of ``max_seq`` triggers promotion, i.e., the
-   counterpart to activation;
-3. Other events trigger similar operations, e.g., offlining an memcg
-   triggers demotion, i.e., the counterpart to deactivation.
-
-In terms of global reclaim, it has two distinct features:
-
-1. Sharding, which allows each thread to start at a random memcg (in
-   the old generation) and improves parallelism;
-2. Eventual fairness, which allows direct reclaim to bail out at will
-   and reduces latency without affecting fairness over some time.
-
-In terms of traversing memcgs during global reclaim, it improves the
-best-case complexity from O(n) to O(1) and does not affect the
-worst-case complexity O(n). Therefore, on average, it has a sublinear
-complexity.
-
 Summary
 -------
 The multi-gen LRU (of folios) can be disassembled into the following
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 75ef7c9f9307..49952301ff3b 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -509,12 +509,6 @@ struct lru_gen_folio {
 	atomic_long_t refaulted[NR_HIST_GENS][ANON_AND_FILE][MAX_NR_TIERS];
 	/* whether the multi-gen LRU is enabled */
 	bool enabled;
-	/* the memcg generation this lru_gen_folio belongs to */
-	u8 gen;
-	/* the list segment this lru_gen_folio belongs to */
-	u8 seg;
-	/* per-node lru_gen_folio list for global reclaim */
-	struct hlist_nulls_node list;
 };
 
 enum {
@@ -558,79 +552,14 @@ struct lru_gen_mm_walk {
 	bool force_scan;
 };
 
-/*
- * For each node, memcgs are divided into two generations: the old and the
- * young. For each generation, memcgs are randomly sharded into multiple bins
- * to improve scalability. For each bin, the hlist_nulls is virtually divided
- * into three segments: the head, the tail and the default.
- *
- * An onlining memcg is added to the tail of a random bin in the old generation.
- * The eviction starts at the head of a random bin in the old generation. The
- * per-node memcg generation counter, whose reminder (mod MEMCG_NR_GENS) indexes
- * the old generation, is incremented when all its bins become empty.
- *
- * There are four operations:
- * 1. MEMCG_LRU_HEAD, which moves a memcg to the head of a random bin in its
- *    current generation (old or young) and updates its "seg" to "head";
- * 2. MEMCG_LRU_TAIL, which moves a memcg to the tail of a random bin in its
- *    current generation (old or young) and updates its "seg" to "tail";
- * 3. MEMCG_LRU_OLD, which moves a memcg to the head of a random bin in the old
- *    generation, updates its "gen" to "old" and resets its "seg" to "default";
- * 4. MEMCG_LRU_YOUNG, which moves a memcg to the tail of a random bin in the
- *    young generation, updates its "gen" to "young" and resets its "seg" to
- *    "default".
- *
- * The events that trigger the above operations are:
- * 1. Exceeding the soft limit, which triggers MEMCG_LRU_HEAD;
- * 2. The first attempt to reclaim a memcg below low, which triggers
- *    MEMCG_LRU_TAIL;
- * 3. The first attempt to reclaim a memcg offlined or below reclaimable size
- *    threshold, which triggers MEMCG_LRU_TAIL;
- * 4. The second attempt to reclaim a memcg offlined or below reclaimable size
- *    threshold, which triggers MEMCG_LRU_YOUNG;
- * 5. Attempting to reclaim a memcg below min, which triggers MEMCG_LRU_YOUNG;
- * 6. Finishing the aging on the eviction path, which triggers MEMCG_LRU_YOUNG;
- * 7. Offlining a memcg, which triggers MEMCG_LRU_OLD.
- *
- * Notes:
- * 1. Memcg LRU only applies to global reclaim, and the round-robin incrementing
- *    of their max_seq counters ensures the eventual fairness to all eligible
- *    memcgs. For memcg reclaim, it still relies on mem_cgroup_iter().
- * 2. There are only two valid generations: old (seq) and young (seq+1).
- *    MEMCG_NR_GENS is set to three so that when reading the generation counter
- *    locklessly, a stale value (seq-1) does not wraparound to young.
- */
-#define MEMCG_NR_GENS	3
-#define MEMCG_NR_BINS	8
-
-struct lru_gen_memcg {
-	/* the per-node memcg generation counter */
-	unsigned long seq;
-	/* each memcg has one lru_gen_folio per node */
-	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];
-	/* protects the above */
-	spinlock_t lock;
-};
-
-void lru_gen_init_pgdat(struct pglist_data *pgdat);
 void lru_gen_init_lruvec(struct lruvec *lruvec);
 bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw);
 
 void lru_gen_init_memcg(struct mem_cgroup *memcg);
 void lru_gen_exit_memcg(struct mem_cgroup *memcg);
-void lru_gen_online_memcg(struct mem_cgroup *memcg);
-void lru_gen_offline_memcg(struct mem_cgroup *memcg);
-void lru_gen_release_memcg(struct mem_cgroup *memcg);
-void lru_gen_soft_reclaim(struct mem_cgroup *memcg, int nid);
 
 #else /* !CONFIG_LRU_GEN */
 
-static inline void lru_gen_init_pgdat(struct pglist_data *pgdat)
-{
-}
-
 static inline void lru_gen_init_lruvec(struct lruvec *lruvec)
 {
 }
@@ -648,22 +577,6 @@ static inline void lru_gen_exit_memcg(struct mem_cgroup *memcg)
 {
 }
 
-static inline void lru_gen_online_memcg(struct mem_cgroup *memcg)
-{
-}
-
-static inline void lru_gen_offline_memcg(struct mem_cgroup *memcg)
-{
-}
-
-static inline void lru_gen_release_memcg(struct mem_cgroup *memcg)
-{
-}
-
-static inline void lru_gen_soft_reclaim(struct mem_cgroup *memcg, int nid)
-{
-}
-
 #endif /* CONFIG_LRU_GEN */
 
 struct lruvec {
@@ -1503,8 +1416,6 @@ typedef struct pglist_data {
 #ifdef CONFIG_LRU_GEN
 	/* kswap mm walk data */
 	struct lru_gen_mm_walk mm_walk;
-	/* lru_gen_folio list */
-	struct lru_gen_memcg memcg_lru;
 #endif
 
 	CACHELINE_PADDING(_pad2_);
diff --git a/mm/memcontrol-v1.c b/mm/memcontrol-v1.c
index 6eed14bff742..8f41e72ae7f0 100644
--- a/mm/memcontrol-v1.c
+++ b/mm/memcontrol-v1.c
@@ -182,12 +182,6 @@ static void memcg1_update_tree(struct mem_cgroup *memcg, int nid)
 	struct mem_cgroup_per_node *mz;
 	struct mem_cgroup_tree_per_node *mctz;
 
-	if (lru_gen_enabled()) {
-		if (soft_limit_excess(memcg))
-			lru_gen_soft_reclaim(memcg, nid);
-		return;
-	}
-
 	mctz = soft_limit_tree.rb_tree_per_node[nid];
 	if (!mctz)
 		return;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index be810c1fbfc3..ab3ebecb5ec7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3874,8 +3874,6 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 	if (unlikely(mem_cgroup_is_root(memcg)) && !mem_cgroup_disabled())
 		queue_delayed_work(system_unbound_wq, &stats_flush_dwork,
 				   FLUSH_TIME);
-	lru_gen_online_memcg(memcg);
-
 	/* Online state pins memcg ID, memcg ID pins CSS */
 	refcount_set(&memcg->id.ref, 1);
 	css_get(css);
@@ -3915,7 +3913,6 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
 	reparent_deferred_split_queue(memcg);
 	reparent_shrinker_deferred(memcg);
 	wb_memcg_offline(memcg);
-	lru_gen_offline_memcg(memcg);
 
 	drain_all_stock(memcg);
 
@@ -3927,7 +3924,6 @@ static void mem_cgroup_css_released(struct cgroup_subsys_state *css)
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
 
 	invalidate_reclaim_iterators(memcg);
-	lru_gen_release_memcg(memcg);
 }
 
 static void mem_cgroup_css_free(struct cgroup_subsys_state *css)
diff --git a/mm/mm_init.c b/mm/mm_init.c
index fc2a6f1e518f..6e5e1fe6ff31 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -1745,7 +1745,6 @@ static void __init free_area_init_node(int nid)
 	pgdat_set_deferred_range(pgdat);
 
 	free_area_init_core(pgdat);
-	lru_gen_init_pgdat(pgdat);
 }
 
 /* Any regular or high memory on that node ? */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 70b0e7e5393c..584f41eb4c14 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2698,9 +2698,6 @@ static bool should_clear_pmd_young(void)
 #define for_each_evictable_type(type, swappiness)			\
 	for ((type) = min_type(swappiness); (type) <= max_type(swappiness); (type)++)
 
-#define get_memcg_gen(seq)	((seq) % MEMCG_NR_GENS)
-#define get_memcg_bin(bin)	((bin) % MEMCG_NR_BINS)
-
 static struct lruvec *get_lruvec(struct mem_cgroup *memcg, int nid)
 {
 	struct pglist_data *pgdat = NODE_DATA(nid);
@@ -4287,140 +4284,6 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 	return true;
 }
 
-/******************************************************************************
- *                          memcg LRU
- ******************************************************************************/
-
-/* see the comment on MEMCG_NR_GENS */
-enum {
-	MEMCG_LRU_NOP,
-	MEMCG_LRU_HEAD,
-	MEMCG_LRU_TAIL,
-	MEMCG_LRU_OLD,
-	MEMCG_LRU_YOUNG,
-};
-
-static void lru_gen_rotate_memcg(struct lruvec *lruvec, int op)
-{
-	int seg;
-	int old, new;
-	unsigned long flags;
-	int bin = get_random_u32_below(MEMCG_NR_BINS);
-	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
-
-	spin_lock_irqsave(&pgdat->memcg_lru.lock, flags);
-
-	VM_WARN_ON_ONCE(hlist_nulls_unhashed(&lruvec->lrugen.list));
-
-	seg = 0;
-	new = old = lruvec->lrugen.gen;
-
-	/* see the comment on MEMCG_NR_GENS */
-	if (op == MEMCG_LRU_HEAD)
-		seg = MEMCG_LRU_HEAD;
-	else if (op == MEMCG_LRU_TAIL)
-		seg = MEMCG_LRU_TAIL;
-	else if (op == MEMCG_LRU_OLD)
-		new = get_memcg_gen(pgdat->memcg_lru.seq);
-	else if (op == MEMCG_LRU_YOUNG)
-		new = get_memcg_gen(pgdat->memcg_lru.seq + 1);
-	else
-		VM_WARN_ON_ONCE(true);
-
-	WRITE_ONCE(lruvec->lrugen.seg, seg);
-	WRITE_ONCE(lruvec->lrugen.gen, new);
-
-	hlist_nulls_del_rcu(&lruvec->lrugen.list);
-
-	if (op == MEMCG_LRU_HEAD || op == MEMCG_LRU_OLD)
-		hlist_nulls_add_head_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[new][bin]);
-	else
-		hlist_nulls_add_tail_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[new][bin]);
-
-	pgdat->memcg_lru.nr_memcgs[old]--;
-	pgdat->memcg_lru.nr_memcgs[new]++;
-
-	if (!pgdat->memcg_lru.nr_memcgs[old] && old == get_memcg_gen(pgdat->memcg_lru.seq))
-		WRITE_ONCE(pgdat->memcg_lru.seq, pgdat->memcg_lru.seq + 1);
-
-	spin_unlock_irqrestore(&pgdat->memcg_lru.lock, flags);
-}
-
-#ifdef CONFIG_MEMCG
-
-void lru_gen_online_memcg(struct mem_cgroup *memcg)
-{
-	int gen;
-	int nid;
-	int bin = get_random_u32_below(MEMCG_NR_BINS);
-
-	for_each_node(nid) {
-		struct pglist_data *pgdat = NODE_DATA(nid);
-		struct lruvec *lruvec = get_lruvec(memcg, nid);
-
-		spin_lock_irq(&pgdat->memcg_lru.lock);
-
-		VM_WARN_ON_ONCE(!hlist_nulls_unhashed(&lruvec->lrugen.list));
-
-		gen = get_memcg_gen(pgdat->memcg_lru.seq);
-
-		lruvec->lrugen.gen = gen;
-
-		hlist_nulls_add_tail_rcu(&lruvec->lrugen.list, &pgdat->memcg_lru.fifo[gen][bin]);
-		pgdat->memcg_lru.nr_memcgs[gen]++;
-
-		spin_unlock_irq(&pgdat->memcg_lru.lock);
-	}
-}
-
-void lru_gen_offline_memcg(struct mem_cgroup *memcg)
-{
-	int nid;
-
-	for_each_node(nid) {
-		struct lruvec *lruvec = get_lruvec(memcg, nid);
-
-		lru_gen_rotate_memcg(lruvec, MEMCG_LRU_OLD);
-	}
-}
-
-void lru_gen_release_memcg(struct mem_cgroup *memcg)
-{
-	int gen;
-	int nid;
-
-	for_each_node(nid) {
-		struct pglist_data *pgdat = NODE_DATA(nid);
-		struct lruvec *lruvec = get_lruvec(memcg, nid);
-
-		spin_lock_irq(&pgdat->memcg_lru.lock);
-
-		if (hlist_nulls_unhashed(&lruvec->lrugen.list))
-			goto unlock;
-
-		gen = lruvec->lrugen.gen;
-
-		hlist_nulls_del_init_rcu(&lruvec->lrugen.list);
-		pgdat->memcg_lru.nr_memcgs[gen]--;
-
-		if (!pgdat->memcg_lru.nr_memcgs[gen] && gen == get_memcg_gen(pgdat->memcg_lru.seq))
-			WRITE_ONCE(pgdat->memcg_lru.seq, pgdat->memcg_lru.seq + 1);
-unlock:
-		spin_unlock_irq(&pgdat->memcg_lru.lock);
-	}
-}
-
-void lru_gen_soft_reclaim(struct mem_cgroup *memcg, int nid)
-{
-	struct lruvec *lruvec = get_lruvec(memcg, nid);
-
-	/* see the comment on MEMCG_NR_GENS */
-	if (READ_ONCE(lruvec->lrugen.seg) != MEMCG_LRU_HEAD)
-		lru_gen_rotate_memcg(lruvec, MEMCG_LRU_HEAD);
-}
-
-#endif /* CONFIG_MEMCG */
-
 /******************************************************************************
  *                          the eviction
  ******************************************************************************/
@@ -5613,18 +5476,6 @@ static const struct file_operations lru_gen_ro_fops = {
  *                          initialization
  ******************************************************************************/
 
-void lru_gen_init_pgdat(struct pglist_data *pgdat)
-{
-	int i, j;
-
-	spin_lock_init(&pgdat->memcg_lru.lock);
-
-	for (i = 0; i < MEMCG_NR_GENS; i++) {
-		for (j = 0; j < MEMCG_NR_BINS; j++)
-			INIT_HLIST_NULLS_HEAD(&pgdat->memcg_lru.fifo[i][j], i);
-	}
-}
-
 void lru_gen_init_lruvec(struct lruvec *lruvec)
 {
 	int i;
@@ -5671,9 +5522,7 @@ void lru_gen_exit_memcg(struct mem_cgroup *memcg)
 		struct lru_gen_mm_state *mm_state = get_mm_state(lruvec);
 
 		VM_WARN_ON_ONCE(memchr_inv(lruvec->lrugen.nr_pages, 0,
-					   sizeof(lruvec->lrugen.nr_pages)));
-
-		lruvec->lrugen.list.next = LIST_POISON1;
+				   sizeof(lruvec->lrugen.nr_pages)));
 
 		if (!mm_state)
 			continue;
-- 
2.34.1

Re: [RFC PATCH -next 1/2] mm/mglru: use mem_cgroup_iter for global reclaim

[Johannes Weiner]

On Thu, Dec 04, 2025 at 12:31:23PM +0000, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.com>
> 
> The memcg LRU was originally introduced for global reclaim to enhance
> scalability. However, its implementation complexity has led to performance
> regressions when dealing with a large number of memory cgroups [1].
> 
> As suggested by Johannes [1], this patch adopts mem_cgroup_iter with
> cookie-based iteration for global reclaim, aligning with the approach
> already used in shrink_node_memcgs. This simplification removes the
> dedicated memcg LRU tracking while maintaining the core functionality.
> 
> It performed a stress test based on Zhao Yu's methodology [2] on a
> 1 TB, 4-node NUMA system. The results are summarized below:
> 
> 					memcg LRU    memcg iter
> stddev(pgsteal) / mean(pgsteal)            91.2%         75.7%
> sum(pgsteal) / sum(requested)             216.4%        230.5%
> 
> The new implementation demonstrates a significant improvement in
> fairness, reducing the standard deviation relative to the mean by
> 15.5 percentage points. While the reclaim accuracy shows a slight
> increase in overscan (from 85086871 to 90633890, 6.5%).
> 
> The primary benefits of this change are:
> 1. Simplified codebase by removing custom memcg LRU infrastructure
> 2. Improved fairness in memory reclaim across multiple cgroups
> 3. Better performance when creating many memory cgroups
> 
> [1] https://lore.kernel.org/r/20251126171513.GC135004@cmpxchg.org
> [2] https://lore.kernel.org/r/20221222041905.2431096-7-yuzhao@google.com
> Signed-off-by: Chen Ridong <chenridong@huawei.com>

Acked-by: Johannes Weiner <hannes@cmxpchg.org>

The diff and the test results look good to me. Comparing the resulting
shrink_many() with shrink_node_memcgs(), this also looks like a great
step towards maintainability and unification.

Thanks!

Re: [RFC PATCH -next 2/2] mm/mglru: remove memcg lru

[Johannes Weiner]

On Thu, Dec 04, 2025 at 12:31:24PM +0000, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.com>
> 
> Now that the previous patch has switched global reclaim to use
> mem_cgroup_iter, the specialized memcg LRU infrastructure is no longer
> needed. This patch removes all related code:
> 
> Signed-off-by: Chen Ridong <chenridong@huawei.com>

Looks good to me!

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

> ---
>  Documentation/mm/multigen_lru.rst |  30 ------
>  include/linux/mmzone.h            |  89 -----------------
>  mm/memcontrol-v1.c                |   6 --
>  mm/memcontrol.c                   |   4 -
>  mm/mm_init.c                      |   1 -
>  mm/vmscan.c                       | 153 +-----------------------------
>  6 files changed, 1 insertion(+), 282 deletions(-)

Re: [RFC PATCH -next 1/2] mm/mglru: use mem_cgroup_iter for global reclaim

[Shakeel Butt]

Hi Chen,

On Thu, Dec 04, 2025 at 12:31:23PM +0000, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.com>
> 
> The memcg LRU was originally introduced for global reclaim to enhance
> scalability. However, its implementation complexity has led to performance
> regressions when dealing with a large number of memory cgroups [1].
> 
> As suggested by Johannes [1], this patch adopts mem_cgroup_iter with
> cookie-based iteration for global reclaim, aligning with the approach
> already used in shrink_node_memcgs. This simplification removes the
> dedicated memcg LRU tracking while maintaining the core functionality.
> 
> It performed a stress test based on Zhao Yu's methodology [2] on a
> 1 TB, 4-node NUMA system. The results are summarized below:
> 
> 					memcg LRU    memcg iter
> stddev(pgsteal) / mean(pgsteal)            91.2%         75.7%
> sum(pgsteal) / sum(requested)             216.4%        230.5%
> 
> The new implementation demonstrates a significant improvement in
> fairness, reducing the standard deviation relative to the mean by
> 15.5 percentage points. While the reclaim accuracy shows a slight
> increase in overscan (from 85086871 to 90633890, 6.5%).
> 
> The primary benefits of this change are:
> 1. Simplified codebase by removing custom memcg LRU infrastructure
> 2. Improved fairness in memory reclaim across multiple cgroups
> 3. Better performance when creating many memory cgroups
> 
> [1] https://lore.kernel.org/r/20251126171513.GC135004@cmpxchg.org
> [2] https://lore.kernel.org/r/20221222041905.2431096-7-yuzhao@google.com
> Signed-off-by: Chen Ridong <chenridong@huawei.com>

Thanks a lot of this awesome work.

> ---
>  mm/vmscan.c | 117 ++++++++++++++++------------------------------------
>  1 file changed, 36 insertions(+), 81 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index fddd168a9737..70b0e7e5393c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4895,27 +4895,14 @@ static bool try_to_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
>  	return nr_to_scan < 0;
>  }
>  
> -static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
> +static void shrink_one(struct lruvec *lruvec, struct scan_control *sc)
>  {
> -	bool success;
>  	unsigned long scanned = sc->nr_scanned;
>  	unsigned long reclaimed = sc->nr_reclaimed;
> -	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>  	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> +	struct mem_cgroup *memcg = lruvec_memcg(lruvec);
>  
> -	/* lru_gen_age_node() called mem_cgroup_calculate_protection() */
> -	if (mem_cgroup_below_min(NULL, memcg))
> -		return MEMCG_LRU_YOUNG;
> -
> -	if (mem_cgroup_below_low(NULL, memcg)) {
> -		/* see the comment on MEMCG_NR_GENS */
> -		if (READ_ONCE(lruvec->lrugen.seg) != MEMCG_LRU_TAIL)
> -			return MEMCG_LRU_TAIL;
> -
> -		memcg_memory_event(memcg, MEMCG_LOW);
> -	}
> -
> -	success = try_to_shrink_lruvec(lruvec, sc);
> +	try_to_shrink_lruvec(lruvec, sc);
>  
>  	shrink_slab(sc->gfp_mask, pgdat->node_id, memcg, sc->priority);
>  
> @@ -4924,86 +4911,55 @@ static int shrink_one(struct lruvec *lruvec, struct scan_control *sc)
>  			   sc->nr_reclaimed - reclaimed);
>  
>  	flush_reclaim_state(sc);

Unrealted to your patch but why this flush_reclaim_state() is at
different place from the non-MGLRU code path?

> -
> -	if (success && mem_cgroup_online(memcg))
> -		return MEMCG_LRU_YOUNG;
> -
> -	if (!success && lruvec_is_sizable(lruvec, sc))
> -		return 0;
> -
> -	/* one retry if offlined or too small */
> -	return READ_ONCE(lruvec->lrugen.seg) != MEMCG_LRU_TAIL ?
> -	       MEMCG_LRU_TAIL : MEMCG_LRU_YOUNG;
>  }
>  
>  static void shrink_many(struct pglist_data *pgdat, struct scan_control *sc)

This function kind of become very similar to shrink_node_memcgs()
function other than shrink_one vs shrink_lruvec. Can you try to combine
them and see if it looks not-ugly? Otherwise the code looks good to me.

Re: [RFC PATCH -next 1/2] mm/mglru: use mem_cgroup_iter for global reclaim

[zhongjinji]

> From: Chen Ridong <chenridong@huawei.com>
> 
> The memcg LRU was originally introduced for global reclaim to enhance
> scalability. However, its implementation complexity has led to performance
> regressions when dealing with a large number of memory cgroups [1].
> 
> As suggested by Johannes [1], this patch adopts mem_cgroup_iter with
> cookie-based iteration for global reclaim, aligning with the approach
> already used in shrink_node_memcgs. This simplification removes the
> dedicated memcg LRU tracking while maintaining the core functionality.
> 
> It performed a stress test based on Zhao Yu's methodology [2] on a
> 1 TB, 4-node NUMA system. The results are summarized below:
> 
> 					memcg LRU    memcg iter
> stddev(pgsteal) / mean(pgsteal)            91.2%         75.7%
> sum(pgsteal) / sum(requested)             216.4%        230.5%

Are there more data available? For example, the load of kswapd or the refault values.

I am concerned about these two data points because Yu Zhao's implementation controls
the fairness of aging through memcg gen (get_memcg_gen). This helps reduce excessive
aging for certain cgroups, which is beneficial for kswapd's power consumption.

At the same time, pages that age earlier can be considered colder pages (in the entire system),
so reclaiming them should also help with the refault values.

> The new implementation demonstrates a significant improvement in
> fairness, reducing the standard deviation relative to the mean by
> 15.5 percentage points. While the reclaim accuracy shows a slight
> increase in overscan (from 85086871 to 90633890, 6.5%).
> 
> The primary benefits of this change are:
> 1. Simplified codebase by removing custom memcg LRU infrastructure
> 2. Improved fairness in memory reclaim across multiple cgroups
> 3. Better performance when creating many memory cgroups
> 
> [1] https://lore.kernel.org/r/20251126171513.GC135004@cmpxchg.org
> [2] https://lore.kernel.org/r/20221222041905.2431096-7-yuzhao@google.com
> Signed-off-by: Chen Ridong <chenridong@huawei.com>