Date:	Wed, 22 Oct 2014 10:40:44 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: remove mem_cgroup_reclaimable check from soft
 reclaim

On Tue, Oct 21, 2014 at 02:22:39PM -0400, Johannes Weiner wrote:
> On Tue, Oct 21, 2014 at 05:15:50PM +0400, Vladimir Davydov wrote:
> > mem_cgroup_reclaimable() checks whether a cgroup has reclaimable pages
> > on *any* NUMA node. However, the only place where it's called is
> > mem_cgroup_soft_reclaim(), which tries to reclaim memory from a
> > *specific* zone. So the way how it's used is incorrect - it will return
> > true even if the cgroup doesn't have pages on the zone we're scanning.
> > 
> > I think we can get rid of this check completely, because
> > mem_cgroup_shrink_node_zone(), which is called by
> > mem_cgroup_soft_reclaim() if mem_cgroup_reclaimable() returns true, is
> > equivalent to shrink_lruvec(), which exits almost immediately if the
> > lruvec passed to it is empty. So there's no need to optimize anything
> > here. Besides, we don't have such a check in the general scan path
> > (shrink_zone) either.
> > 
> > Signed-off-by: Vladimir Davydov <vdavydov@...allels.com>
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> 
> How about this on top?
> 
> ---
> 
> From 27bd24b00433d9f6c8d60ba2b13dbff158b06c13 Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@...xchg.org>
> Date: Tue, 21 Oct 2014 09:53:54 -0400
> Subject: [patch] mm: memcontrol: do not filter reclaimable nodes in NUMA
>  round-robin
> 
> The round-robin node reclaim currently tries to include only nodes
> that have memory of the memcg in question, which is quite elaborate.
> 
> Just use plain round-robin over the nodes that are allowed by the
> task's cpuset, which are the most likely to contain that memcg's
> memory.  But even if zones without memcg memory are encountered,
> direct reclaim will skip over them without too much hassle.
> 
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>

Totally agree.

Acked-by: Vladimir Davydov <vdavydov@...allels.com>

> ---
>  mm/memcontrol.c | 97 +++------------------------------------------------------
>  1 file changed, 5 insertions(+), 92 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d353d9e1fdca..293db8234179 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -54,6 +54,7 @@
>  #include <linux/page_cgroup.h>
>  #include <linux/cpu.h>
>  #include <linux/oom.h>
> +#include <linux/cpuset.h>
>  #include <linux/lockdep.h>
>  #include <linux/file.h>
>  #include "internal.h"
> @@ -129,12 +130,10 @@ static const char * const mem_cgroup_lru_names[] = {
>  enum mem_cgroup_events_target {
>  	MEM_CGROUP_TARGET_THRESH,
>  	MEM_CGROUP_TARGET_SOFTLIMIT,
> -	MEM_CGROUP_TARGET_NUMAINFO,
>  	MEM_CGROUP_NTARGETS,
>  };
>  #define THRESHOLDS_EVENTS_TARGET 128
>  #define SOFTLIMIT_EVENTS_TARGET 1024
> -#define NUMAINFO_EVENTS_TARGET	1024
>  
>  struct mem_cgroup_stat_cpu {
>  	long count[MEM_CGROUP_STAT_NSTATS];
> @@ -352,11 +351,6 @@ struct mem_cgroup {
>  #endif
>  
>  	int last_scanned_node;
> -#if MAX_NUMNODES > 1
> -	nodemask_t	scan_nodes;
> -	atomic_t	numainfo_events;
> -	atomic_t	numainfo_updating;
> -#endif
>  
>  	/* List of events which userspace want to receive */
>  	struct list_head event_list;
> @@ -965,9 +959,6 @@ static bool mem_cgroup_event_ratelimit(struct mem_cgroup *memcg,
>  		case MEM_CGROUP_TARGET_SOFTLIMIT:
>  			next = val + SOFTLIMIT_EVENTS_TARGET;
>  			break;
> -		case MEM_CGROUP_TARGET_NUMAINFO:
> -			next = val + NUMAINFO_EVENTS_TARGET;
> -			break;
>  		default:
>  			break;
>  		}
> @@ -986,22 +977,10 @@ static void memcg_check_events(struct mem_cgroup *memcg, struct page *page)
>  	/* threshold event is triggered in finer grain than soft limit */
>  	if (unlikely(mem_cgroup_event_ratelimit(memcg,
>  						MEM_CGROUP_TARGET_THRESH))) {
> -		bool do_softlimit;
> -		bool do_numainfo __maybe_unused;
> -
> -		do_softlimit = mem_cgroup_event_ratelimit(memcg,
> -						MEM_CGROUP_TARGET_SOFTLIMIT);
> -#if MAX_NUMNODES > 1
> -		do_numainfo = mem_cgroup_event_ratelimit(memcg,
> -						MEM_CGROUP_TARGET_NUMAINFO);
> -#endif
>  		mem_cgroup_threshold(memcg);
> -		if (unlikely(do_softlimit))
> +		if (mem_cgroup_event_ratelimit(memcg,
> +					       MEM_CGROUP_TARGET_SOFTLIMIT))
>  			mem_cgroup_update_tree(memcg, page);
> -#if MAX_NUMNODES > 1
> -		if (unlikely(do_numainfo))
> -			atomic_inc(&memcg->numainfo_events);
> -#endif
>  	}
>  }
>  
> @@ -1708,61 +1687,7 @@ static void mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
>  			 NULL, "Memory cgroup out of memory");
>  }
>  
> -/**
> - * test_mem_cgroup_node_reclaimable
> - * @memcg: the target memcg
> - * @nid: the node ID to be checked.
> - * @noswap : specify true here if the user wants flle only information.
> - *
> - * This function returns whether the specified memcg contains any
> - * reclaimable pages on a node. Returns true if there are any reclaimable
> - * pages in the node.
> - */
> -static bool test_mem_cgroup_node_reclaimable(struct mem_cgroup *memcg,
> -		int nid, bool noswap)
> -{
> -	if (mem_cgroup_node_nr_lru_pages(memcg, nid, LRU_ALL_FILE))
> -		return true;
> -	if (noswap || !total_swap_pages)
> -		return false;
> -	if (mem_cgroup_node_nr_lru_pages(memcg, nid, LRU_ALL_ANON))
> -		return true;
> -	return false;
> -
> -}
>  #if MAX_NUMNODES > 1
> -
> -/*
> - * Always updating the nodemask is not very good - even if we have an empty
> - * list or the wrong list here, we can start from some node and traverse all
> - * nodes based on the zonelist. So update the list loosely once per 10 secs.
> - *
> - */
> -static void mem_cgroup_may_update_nodemask(struct mem_cgroup *memcg)
> -{
> -	int nid;
> -	/*
> -	 * numainfo_events > 0 means there was at least NUMAINFO_EVENTS_TARGET
> -	 * pagein/pageout changes since the last update.
> -	 */
> -	if (!atomic_read(&memcg->numainfo_events))
> -		return;
> -	if (atomic_inc_return(&memcg->numainfo_updating) > 1)
> -		return;
> -
> -	/* make a nodemask where this memcg uses memory from */
> -	memcg->scan_nodes = node_states[N_MEMORY];
> -
> -	for_each_node_mask(nid, node_states[N_MEMORY]) {
> -
> -		if (!test_mem_cgroup_node_reclaimable(memcg, nid, false))
> -			node_clear(nid, memcg->scan_nodes);
> -	}
> -
> -	atomic_set(&memcg->numainfo_events, 0);
> -	atomic_set(&memcg->numainfo_updating, 0);
> -}
> -
>  /*
>   * Selecting a node where we start reclaim from. Because what we need is just
>   * reducing usage counter, start from anywhere is O,K. Considering
> @@ -1779,21 +1704,9 @@ int mem_cgroup_select_victim_node(struct mem_cgroup *memcg)
>  {
>  	int node;
>  
> -	mem_cgroup_may_update_nodemask(memcg);
> -	node = memcg->last_scanned_node;
> -
> -	node = next_node(node, memcg->scan_nodes);
> +	node = next_node(memcg->last_scanned_node, cpuset_current_mems_allowed);
>  	if (node == MAX_NUMNODES)
> -		node = first_node(memcg->scan_nodes);
> -	/*
> -	 * We call this when we hit limit, not when pages are added to LRU.
> -	 * No LRU may hold pages because all pages are UNEVICTABLE or
> -	 * memcg is too small and all pages are not on LRU. In that case,
> -	 * we use curret node.
> -	 */
> -	if (unlikely(node == MAX_NUMNODES))
> -		node = numa_node_id();
> -
> +		node = first_node(cpuset_current_mems_allowed);
>  	memcg->last_scanned_node = node;
>  	return node;
>  }
> -- 
> 2.1.2
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