Re: [PATCH V8 2/2] mm: memcg detect no memcgs above softlimit under zone reclaim

[Michal Hocko <mhocko@suse.cz>]

On Thu 02-08-12 14:24:24, Ying Han wrote:
> In memcg kernel, cgroup under its softlimit is not targeted under global
> reclaim. It could be possible that all memcgs are under their softlimit for
> a particular zone. If that is the case, the current implementation will
> burn extra cpu cycles without making forward progress.
> 
> The idea is from LSF discussion where we detect it after the first round of
> scanning and restart the reclaim by not looking at softlimit at all. This
> allows us to make forward progress on shrink_zone().
> 
> Signed-off-by: Ying Han <yinghan@google.com>
> ---
>  include/linux/memcontrol.h |    9 +++++++++
>  mm/memcontrol.c            |    3 +--
>  mm/vmscan.c                |   18 ++++++++++++++++--
>  3 files changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 65538f9..cbad102 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -180,6 +180,8 @@ static inline void mem_cgroup_dec_page_stat(struct page *page,
>  }
>  
>  void mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx);
> +
> +bool mem_cgroup_is_root(struct mem_cgroup *memcg);
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  void mem_cgroup_split_huge_fixup(struct page *head);
>  #endif
> @@ -360,6 +362,13 @@ static inline
>  void mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx)
>  {
>  }
> +
> +static inline bool
> +mem_cgroup_is_root(struct mem_cgroup *memcg)
> +{
> +	return true;
> +}
> +
>  static inline void mem_cgroup_replace_page_cache(struct page *oldpage,
>  				struct page *newpage)
>  {
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d8b91bb..368eecc 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -378,7 +378,6 @@ enum charge_type {
>  
>  static void mem_cgroup_get(struct mem_cgroup *memcg);
>  static void mem_cgroup_put(struct mem_cgroup *memcg);
> -static bool mem_cgroup_is_root(struct mem_cgroup *memcg);
>  
>  static inline
>  struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *s)
> @@ -850,7 +849,7 @@ void mem_cgroup_iter_break(struct mem_cgroup *root,
>  	     iter != NULL;				\
>  	     iter = mem_cgroup_iter(NULL, iter, NULL))
>  
> -static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
> +bool mem_cgroup_is_root(struct mem_cgroup *memcg)
>  {
>  	return (memcg == root_mem_cgroup);
>  }
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 88487b3..8622022 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1861,6 +1861,10 @@ static void shrink_zone(struct zone *zone, struct scan_control *sc)
>  		.priority = sc->priority,
>  	};
>  	struct mem_cgroup *memcg;
> +	bool over_softlimit, ignore_softlimit = false;
> +
> +restart:
> +	over_softlimit = false;
>  
>  	memcg = mem_cgroup_iter(root, NULL, &reclaim);
>  	do {
> @@ -1879,10 +1883,15 @@ static void shrink_zone(struct zone *zone, struct scan_control *sc)
>  		 * we have to reclaim under softlimit instead of burning more
>  		 * cpu cycles.
>  		 */
> -		if (!global_reclaim(sc) || sc->priority < DEF_PRIORITY ||
> -				mem_cgroup_over_soft_limit(memcg))
> +		if (ignore_softlimit || !global_reclaim(sc) ||
> +				sc->priority < DEF_PRIORITY ||
> +				mem_cgroup_over_soft_limit(memcg)) {
>  			shrink_lruvec(lruvec, sc);
>  
> +			if (!mem_cgroup_is_root(memcg))
> +				over_softlimit = true;
> +		}
> +

I think this is still not sufficient because you do not want to hammer
root in the ignore_softlimit case. 
-- 
Michal Hocko
SUSE Labs

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