Re: [PATCH V7 2/2] mm: memcg detect no memcgs above softlimit under zone reclaim

[Michal Hocko <mhocko@suse.cz>]

On Mon 30-07-12 15:32:18, 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. 

This is a bit misleading because there is no softlimit per zone...

> If that is the case, the current implementation will burn extra cpu
> cycles without making forward progress.

This scales with the number of groups which is bareable I guess. We do
not drop priority so the wasted round will not make a bigger pressure on
the reclaim.

> 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>
> ---
>  mm/vmscan.c |   17 +++++++++++++++--
>  1 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 59e633c..747d903 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 +1883,14 @@ 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 - 2 ||
> -				mem_cgroup_over_soft_limit(memcg))
> +		if (ignore_softlimit || !global_reclaim(sc) ||
> +				sc->priority < DEF_PRIORITY - 2 ||
> +				mem_cgroup_over_soft_limit(memcg)) {
>  			shrink_lruvec(lruvec, sc);
>  
> +			over_softlimit = true;
> +		}
> +
>  		/*
>  		 * Limit reclaim has historically picked one memcg and
>  		 * scanned it with decreasing priority levels until
> @@ -1899,6 +1907,11 @@ static void shrink_zone(struct zone *zone, struct scan_control *sc)
>  		}
>  		memcg = mem_cgroup_iter(root, memcg, &reclaim);
>  	} while (memcg);
> +
> +	if (!over_softlimit) {

Is this ever false? At least root cgroup is always above the limit.
Shouldn't we rather compare reclaimed pages?

> +		ignore_softlimit = true;
> +		goto restart;
> +	}
>  }
>  
>  /* Returns true if compaction should go ahead for a high-order request */
> -- 
> 1.7.7.3
> 

-- 
Michal Hocko
SUSE Labs

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