Re: [patch 04/10] mm: memcg: per-priority per-zone hierarchy scangenerations

[Michal Hocko]

[linux-foundation.org seems to be down, so I have changed Andrew's
address]

On Thu 29-09-11 23:00:58, Johannes Weiner wrote:
> Memory cgroup limit reclaim currently picks one memory cgroup out of
> the target hierarchy, remembers it as the last scanned child, and
> reclaims all zones in it with decreasing priority levels.
> 
> The new hierarchy reclaim code will pick memory cgroups from the same
> hierarchy concurrently from different zones and priority levels, it
> becomes necessary that hierarchy roots not only remember the last
> scanned child, but do so for each zone and priority level.
> 
> Until now, we reclaimed memcgs like this:
> 
>     mem = mem_cgroup_iter(root)
>     for each priority level:
>       for each zone in zonelist:
>         reclaim(mem, zone)
> 
> But subsequent patches will move the memcg iteration inside the loop
> over the zones:
> 
>     for each priority level:
>       for each zone in zonelist:
>         mem = mem_cgroup_iter(root)
>         reclaim(mem, zone)
> 
> And to keep with the original scan order - memcg -> priority -> zone -
> the last scanned memcg has to be remembered per zone and per priority
> level.
> 
> Furthermore, global reclaim will be switched to the hierarchy walk as
> well.  Different from limit reclaim, which can just recheck the limit
> after some reclaim progress, its target is to scan all memcgs for the
> desired zone pages, proportional to the memcg size, and so reliably
> detecting a full hierarchy round-trip will become crucial.
> 
> Currently, the code relies on one reclaimer encountering the same
> memcg twice, but that is error-prone with concurrent reclaimers.
> Instead, use a generation counter that is increased every time the
> child with the highest ID has been visited, so that reclaimers can
> stop when the generation changes.

Both naming and the patch description is much nicer. Thanks!

> 
> Signed-off-by: Johannes Weiner <jweiner@xxxxxxxxxx>
> Reviewed-by: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>

Reviewed-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  mm/memcontrol.c |   65 +++++++++++++++++++++++++++++++++++++++---------------
>  1 files changed, 47 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0ba59f6..38d195d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -121,6 +121,13 @@ struct mem_cgroup_stat_cpu {
>  	unsigned long targets[MEM_CGROUP_NTARGETS];
>  };
>  
> +struct mem_cgroup_reclaim_iter {
> +	/* css_id of the last scanned hierarchy member */
> +	int position;
> +	/* scan generation, increased every round-trip */
> +	unsigned int generation;
> +};
> +
>  /*
>   * per-zone information in memory controller.
>   */
> @@ -131,6 +138,8 @@ struct mem_cgroup_per_zone {
>  	struct list_head	lists[NR_LRU_LISTS];
>  	unsigned long		count[NR_LRU_LISTS];
>  
> +	struct mem_cgroup_reclaim_iter reclaim_iter[DEF_PRIORITY + 1];
> +
>  	struct zone_reclaim_stat reclaim_stat;
>  	struct rb_node		tree_node;	/* RB tree node */
>  	unsigned long long	usage_in_excess;/* Set to the value by which */
> @@ -231,11 +240,6 @@ struct mem_cgroup {
>  	 * per zone LRU lists.
>  	 */
>  	struct mem_cgroup_lru_info info;
> -	/*
> -	 * While reclaiming in a hierarchy, we cache the last child we
> -	 * reclaimed from.
> -	 */
> -	int last_scanned_child;
>  	int last_scanned_node;
>  #if MAX_NUMNODES > 1
>  	nodemask_t	scan_nodes;
> @@ -781,9 +785,16 @@ struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
>  	return memcg;
>  }
>  
> -static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> -					  struct mem_cgroup *prev,
> -					  bool reclaim)
> +struct mem_cgroup_reclaim_cookie {
> +	struct zone *zone;
> +	int priority;
> +	unsigned int generation;
> +};
> +
> +static struct mem_cgroup *
> +mem_cgroup_iter(struct mem_cgroup *root,
> +		struct mem_cgroup *prev,
> +		struct mem_cgroup_reclaim_cookie *reclaim)
>  {
>  	struct mem_cgroup *mem = NULL;
>  	int id = 0;
> @@ -804,10 +815,20 @@ static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>  	}
>  
>  	while (!mem) {
> +		struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
>  		struct cgroup_subsys_state *css;
>  
> -		if (reclaim)
> -			id = root->last_scanned_child;
> +		if (reclaim) {
> +			int nid = zone_to_nid(reclaim->zone);
> +			int zid = zone_idx(reclaim->zone);
> +			struct mem_cgroup_per_zone *mz;
> +
> +			mz = mem_cgroup_zoneinfo(root, nid, zid);
> +			iter = &mz->reclaim_iter[reclaim->priority];
> +			if (prev && reclaim->generation != iter->generation)
> +				return NULL;
> +			id = iter->position;
> +		}
>  
>  		rcu_read_lock();
>  		css = css_get_next(&mem_cgroup_subsys, id + 1, &root->css, &id);
> @@ -818,8 +839,13 @@ static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>  			id = 0;
>  		rcu_read_unlock();
>  
> -		if (reclaim)
> -			root->last_scanned_child = id;
> +		if (reclaim) {
> +			iter->position = id;
> +			if (!css)
> +				iter->generation++;
> +			else if (!prev && mem)
> +				reclaim->generation = iter->generation;
> +		}
>  
>  		if (prev && !css)
>  			return NULL;
> @@ -842,14 +868,14 @@ static void mem_cgroup_iter_break(struct mem_cgroup *root,
>   * be used for reference counting.
>   */
>  #define for_each_mem_cgroup_tree(iter, root)		\
> -	for (iter = mem_cgroup_iter(root, NULL, false);	\
> +	for (iter = mem_cgroup_iter(root, NULL, NULL);	\
>  	     iter != NULL;				\
> -	     iter = mem_cgroup_iter(root, iter, false))
> +	     iter = mem_cgroup_iter(root, iter, NULL))
>  
>  #define for_each_mem_cgroup(iter)			\
> -	for (iter = mem_cgroup_iter(NULL, NULL, false);	\
> +	for (iter = mem_cgroup_iter(NULL, NULL, NULL);	\
>  	     iter != NULL;				\
> -	     iter = mem_cgroup_iter(NULL, iter, false))
> +	     iter = mem_cgroup_iter(NULL, iter, NULL))
>  
>  static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
>  {
> @@ -1619,6 +1645,10 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_memcg,
>  	bool check_soft = reclaim_options & MEM_CGROUP_RECLAIM_SOFT;
>  	unsigned long excess;
>  	unsigned long nr_scanned;
> +	struct mem_cgroup_reclaim_cookie reclaim = {
> +		.zone = zone,
> +		.priority = 0,
> +	};
>  
>  	excess = res_counter_soft_limit_excess(&root_memcg->res) >> PAGE_SHIFT;
>  
> @@ -1627,7 +1657,7 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_memcg,
>  		noswap = true;
>  
>  	while (1) {
> -		victim = mem_cgroup_iter(root_memcg, victim, true);
> +		victim = mem_cgroup_iter(root_memcg, victim, &reclaim);
>  		if (!victim) {
>  			loop++;
>  			/*
> @@ -4878,7 +4908,6 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
>  		res_counter_init(&memcg->res, NULL);
>  		res_counter_init(&memcg->memsw, NULL);
>  	}
> -	memcg->last_scanned_child = 0;
>  	memcg->last_scanned_node = MAX_NUMNODES;
>  	INIT_LIST_HEAD(&memcg->oom_notify);
>  
> -- 
> 1.7.6.2
> 
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-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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