Re: [PATCH 8/8] mm: vmscan: harmonize writeback congestion tracking for nodes & memcgs

[Roman Gushchin <guro@fb.com>]

On Tue, Oct 22, 2019 at 10:48:03AM -0400, Johannes Weiner wrote:
> The current writeback congestion tracking has separate flags for
> kswapd reclaim (node level) and cgroup limit reclaim (memcg-node
> level). This is unnecessarily complicated: the lruvec is an existing
> abstraction layer for that node-memcg intersection.
> 
> Introduce lruvec->flags and LRUVEC_CONGESTED. Then track that at the
> reclaim root level, which is either the NUMA node for global reclaim,
> or the cgroup-node intersection for cgroup reclaim.

Good idea!

Reviewed-by: Roman Gushchin <guro@fb.com>

> 
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  include/linux/memcontrol.h |  6 +--
>  include/linux/mmzone.h     | 11 ++++--
>  mm/vmscan.c                | 80 ++++++++++++--------------------------
>  3 files changed, 36 insertions(+), 61 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 498cea07cbb1..d8ffcf60440c 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -133,9 +133,6 @@ struct mem_cgroup_per_node {
>  	unsigned long		usage_in_excess;/* Set to the value by which */
>  						/* the soft limit is exceeded*/
>  	bool			on_tree;
> -	bool			congested;	/* memcg has many dirty pages */
> -						/* backed by a congested BDI */
> -
>  	struct mem_cgroup	*memcg;		/* Back pointer, we cannot */
>  						/* use container_of	   */
>  };
> @@ -412,6 +409,9 @@ static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
>  		goto out;
>  	}
>  
> +	if (!memcg)
> +		memcg = root_mem_cgroup;
> +
>  	mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
>  	lruvec = &mz->lruvec;
>  out:
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 449a44171026..c04b4c1f01fa 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -296,6 +296,12 @@ struct zone_reclaim_stat {
>  	unsigned long		recent_scanned[2];
>  };
>  
> +enum lruvec_flags {
> +	LRUVEC_CONGESTED,		/* lruvec has many dirty pages
> +					 * backed by a congested BDI
> +					 */
> +};
> +
>  struct lruvec {
>  	struct list_head		lists[NR_LRU_LISTS];
>  	struct zone_reclaim_stat	reclaim_stat;
> @@ -303,6 +309,8 @@ struct lruvec {
>  	atomic_long_t			inactive_age;
>  	/* Refaults at the time of last reclaim cycle */
>  	unsigned long			refaults;
> +	/* Various lruvec state flags (enum lruvec_flags) */
> +	unsigned long			flags;
>  #ifdef CONFIG_MEMCG
>  	struct pglist_data *pgdat;
>  #endif
> @@ -572,9 +580,6 @@ struct zone {
>  } ____cacheline_internodealigned_in_smp;
>  
>  enum pgdat_flags {
> -	PGDAT_CONGESTED,		/* pgdat has many dirty pages backed by
> -					 * a congested BDI
> -					 */
>  	PGDAT_DIRTY,			/* reclaim scanning has recently found
>  					 * many dirty file pages at the tail
>  					 * of the LRU.
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 65baa89740dd..3e21166d5198 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -267,29 +267,6 @@ static bool writeback_throttling_sane(struct scan_control *sc)
>  #endif
>  	return false;
>  }
> -
> -static void set_memcg_congestion(pg_data_t *pgdat,
> -				struct mem_cgroup *memcg,
> -				bool congested)
> -{
> -	struct mem_cgroup_per_node *mn;
> -
> -	if (!memcg)
> -		return;
> -
> -	mn = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> -	WRITE_ONCE(mn->congested, congested);
> -}
> -
> -static bool memcg_congested(pg_data_t *pgdat,
> -			struct mem_cgroup *memcg)
> -{
> -	struct mem_cgroup_per_node *mn;
> -
> -	mn = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> -	return READ_ONCE(mn->congested);
> -
> -}
>  #else
>  static int prealloc_memcg_shrinker(struct shrinker *shrinker)
>  {
> @@ -309,18 +286,6 @@ static bool writeback_throttling_sane(struct scan_control *sc)
>  {
>  	return true;
>  }
> -
> -static inline void set_memcg_congestion(struct pglist_data *pgdat,
> -				struct mem_cgroup *memcg, bool congested)
> -{
> -}
> -
> -static inline bool memcg_congested(struct pglist_data *pgdat,
> -			struct mem_cgroup *memcg)
> -{
> -	return false;
> -
> -}
>  #endif
>  
>  /*
> @@ -2716,12 +2681,6 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
>  	return inactive_lru_pages > pages_for_compaction;
>  }
>  
> -static bool pgdat_memcg_congested(pg_data_t *pgdat, struct mem_cgroup *memcg)
> -{
> -	return test_bit(PGDAT_CONGESTED, &pgdat->flags) ||
> -		(memcg && memcg_congested(pgdat, memcg));
> -}
> -
>  static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>  {
>  	struct mem_cgroup *root = sc->target_mem_cgroup;
> @@ -2785,8 +2744,11 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  	struct reclaim_state *reclaim_state = current->reclaim_state;
>  	struct mem_cgroup *root = sc->target_mem_cgroup;
>  	unsigned long nr_reclaimed, nr_scanned;
> +	struct lruvec *target_lruvec;
>  	bool reclaimable = false;
>  
> +	target_lruvec = mem_cgroup_lruvec(sc->target_mem_cgroup, pgdat);
> +
>  again:
>  	memset(&sc->nr, 0, sizeof(sc->nr));
>  
> @@ -2829,14 +2791,6 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  		if (sc->nr.writeback && sc->nr.writeback == sc->nr.taken)
>  			set_bit(PGDAT_WRITEBACK, &pgdat->flags);
>  
> -		/*
> -		 * Tag a node as congested if all the dirty pages
> -		 * scanned were backed by a congested BDI and
> -		 * wait_iff_congested will stall.
> -		 */
> -		if (sc->nr.dirty && sc->nr.dirty == sc->nr.congested)
> -			set_bit(PGDAT_CONGESTED, &pgdat->flags);
> -
>  		/* Allow kswapd to start writing pages during reclaim.*/
>  		if (sc->nr.unqueued_dirty == sc->nr.file_taken)
>  			set_bit(PGDAT_DIRTY, &pgdat->flags);
> @@ -2852,12 +2806,17 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  	}
>  
>  	/*
> +	 * Tag a node/memcg as congested if all the dirty pages
> +	 * scanned were backed by a congested BDI and
> +	 * wait_iff_congested will stall.
> +	 *
>  	 * Legacy memcg will stall in page writeback so avoid forcibly
>  	 * stalling in wait_iff_congested().
>  	 */
> -	if (cgroup_reclaim(sc) && writeback_throttling_sane(sc) &&
> +	if ((current_is_kswapd() ||
> +	     (cgroup_reclaim(sc) && writeback_throttling_sane(sc))) &&
>  	    sc->nr.dirty && sc->nr.dirty == sc->nr.congested)
> -		set_memcg_congestion(pgdat, root, true);
> +		set_bit(LRUVEC_CONGESTED, &target_lruvec->flags);
>  
>  	/*
>  	 * Stall direct reclaim for IO completions if underlying BDIs
> @@ -2865,8 +2824,9 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  	 * starts encountering unqueued dirty pages or cycling through
>  	 * the LRU too quickly.
>  	 */
> -	if (!sc->hibernation_mode && !current_is_kswapd() &&
> -	    current_may_throttle() && pgdat_memcg_congested(pgdat, root))
> +	if (!current_is_kswapd() && current_may_throttle() &&
> +	    !sc->hibernation_mode &&
> +	    test_bit(LRUVEC_CONGESTED, &target_lruvec->flags))
>  		wait_iff_congested(BLK_RW_ASYNC, HZ/10);
>  
>  	if (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
> @@ -3080,8 +3040,16 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>  		if (zone->zone_pgdat == last_pgdat)
>  			continue;
>  		last_pgdat = zone->zone_pgdat;
> +
>  		snapshot_refaults(sc->target_mem_cgroup, zone->zone_pgdat);
> -		set_memcg_congestion(last_pgdat, sc->target_mem_cgroup, false);
> +
> +		if (cgroup_reclaim(sc)) {
> +			struct lruvec *lruvec;
> +
> +			lruvec = mem_cgroup_lruvec(sc->target_mem_cgroup,
> +						   zone->zone_pgdat);
> +			clear_bit(LRUVEC_CONGESTED, &lruvec->flags);
> +		}
>  	}
>  
>  	delayacct_freepages_end();
> @@ -3461,7 +3429,9 @@ static bool pgdat_balanced(pg_data_t *pgdat, int order, int classzone_idx)
>  /* Clear pgdat state for congested, dirty or under writeback. */
>  static void clear_pgdat_congested(pg_data_t *pgdat)
>  {
> -	clear_bit(PGDAT_CONGESTED, &pgdat->flags);
> +	struct lruvec *lruvec = mem_cgroup_lruvec(NULL, pgdat);
> +
> +	clear_bit(LRUVEC_CONGESTED, &lruvec->flags);
>  	clear_bit(PGDAT_DIRTY, &pgdat->flags);
>  	clear_bit(PGDAT_WRITEBACK, &pgdat->flags);
>  }
> -- 
> 2.23.0
> 
>