Re: [PATCH 6/9] memcg: remove lru_gen_soft_reclaim()

[Michal Hocko <mhocko@suse.com>]

On Tue 11-08-26 13:32:00, Shakeel Butt wrote:
> 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>

I am not deeply familiar with MGLRU to give my ack but from a quick look
this looks right.

> ---
>  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 94f9c3ff5416..01fabd0ece0d 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -635,35 +635,32 @@ 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
> @@ -696,7 +693,6 @@ 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);
> @@ -737,10 +733,6 @@ 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 790b50c78a2e..71244cf33d59 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

-- 
Michal Hocko
SUSE Labs