[PATCH v2 5/8] memcg: remove lru_gen_soft_reclaim()

[Shakeel Butt <shakeel.butt@linux.dev>]

The soft limit rbtree was the only caller. Dropping it leaves
MEMCG_LRU_HEAD unreachable, since nothing else ever rotates a memcg with
that op, so remove the op too and update the memcg LRU comment.

Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 include/linux/mmzone.h | 30 +++++++++++-------------------
 mm/vmscan.c            | 16 ++--------------
 2 files changed, 13 insertions(+), 33 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 67c84a8a7258..84e237f2c17d 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -638,35 +638,32 @@ struct lru_gen_mm_walk {
  * 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.
+ * into two segments: 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
+ * There are three operations:
+ * 1. 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
+ * 2. 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
+ * 3. 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
+ * 1. 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
+ * 2. 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
+ * 3. 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.
+ * 4. Attempting to reclaim a memcg below min, which triggers MEMCG_LRU_YOUNG;
+ * 5. Finishing the aging on the eviction path, which triggers MEMCG_LRU_YOUNG;
+ * 6. Offlining a memcg, which triggers MEMCG_LRU_OLD.
  *
  * Notes:
  * 1. Memcg LRU only applies to global reclaim, and the round-robin incrementing
@@ -699,7 +696,6 @@ 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);
 void max_lru_gen_memcg(struct mem_cgroup *memcg, int nid);
 bool recheck_lru_gen_max_memcg(struct mem_cgroup *memcg, int nid);
 void lru_gen_reparent_memcg(struct mem_cgroup *memcg, struct mem_cgroup *parent, int nid);
@@ -740,10 +736,6 @@ static inline void lru_gen_release_memcg(struct mem_cgroup *memcg)
 {
 }
 
-static inline void lru_gen_soft_reclaim(struct mem_cgroup *memcg, int nid)
-{
-}
-
 static inline void max_lru_gen_memcg(struct mem_cgroup *memcg, int nid)
 {
 }
diff --git a/mm/vmscan.c b/mm/vmscan.c
index d66b5cd167d6..deb087c57007 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4373,7 +4373,6 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw, unsigned int nr)
 /* see the comment on MEMCG_NR_GENS */
 enum {
 	MEMCG_LRU_NOP,
-	MEMCG_LRU_HEAD,
 	MEMCG_LRU_TAIL,
 	MEMCG_LRU_OLD,
 	MEMCG_LRU_YOUNG,
@@ -4395,9 +4394,7 @@ static void lru_gen_rotate_memcg(struct lruvec *lruvec, int op)
 	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)
+	if (op == MEMCG_LRU_TAIL)
 		seg = MEMCG_LRU_TAIL;
 	else if (op == MEMCG_LRU_OLD)
 		new = get_memcg_gen(pgdat->memcg_lru.seq);
@@ -4411,7 +4408,7 @@ static void lru_gen_rotate_memcg(struct lruvec *lruvec, int op)
 
 	hlist_nulls_del_rcu(&lruvec->lrugen.list);
 
-	if (op == MEMCG_LRU_HEAD || op == MEMCG_LRU_OLD)
+	if (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]);
@@ -4489,15 +4486,6 @@ void lru_gen_release_memcg(struct mem_cgroup *memcg)
 	}
 }
 
-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);
-}
-
 bool recheck_lru_gen_max_memcg(struct mem_cgroup *memcg, int nid)
 {
 	struct lruvec *lruvec = get_lruvec(memcg, nid);
-- 
2.53.0-Meta