Re: [PATCH] vmscan: force scan offline memory cgroups

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Jan 08, 2015 at 05:51:09PM +0300, Vladimir Davydov wrote:
> Since commit b2052564e66d ("mm: memcontrol: continue cache reclaim from
> offlined groups") pages charged to a memory cgroup are not reparented
> when the cgroup is removed. Instead, they are supposed to be reclaimed
> in a regular way, along with pages accounted to online memory cgroups.
> 
> However, an lruvec of an offline memory cgroup will sooner or later get
> so small that it will be scanned only at low scan priorities (see
> get_scan_count()). Therefore, if there are enough reclaimable pages in
> big lruvecs, pages accounted to offline memory cgroups will never be
> scanned at all, wasting memory.
> 
> Fix this by unconditionally forcing scanning dead lruvecs from kswapd.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>

Yes, it makes sense to continue draining them at this point.  I just
have a few comments inline:

> @@ -1367,6 +1367,20 @@ int mem_cgroup_inactive_anon_is_low(struct lruvec *lruvec)
>  	return inactive * inactive_ratio < active;
>  }
>  
> +bool mem_cgroup_need_force_scan(struct lruvec *lruvec)
> +{
> +	struct mem_cgroup_per_zone *mz;
> +	struct mem_cgroup *memcg;
> +
> +	if (mem_cgroup_disabled())
> +		return false;
> +
> +	mz = container_of(lruvec, struct mem_cgroup_per_zone, lruvec);
> +	memcg = mz->memcg;
> +
> +	return !(memcg->css.flags & CSS_ONLINE);
> +}

It's better to name functions after what they do, rather than what
they are used for, to make reuse easy.  mem_cgroup_lruvec_online()?

> @@ -1935,7 +1935,8 @@ static void get_scan_count(struct lruvec *lruvec, int swappiness,
>  	 * latencies, so it's better to scan a minimum amount there as
>  	 * well.
>  	 */
> -	if (current_is_kswapd() && !zone_reclaimable(zone))
> +	if (current_is_kswapd() &&
> +	    (!zone_reclaimable(zone) || mem_cgroup_need_force_scan(lruvec)))
>  		force_scan = true;

This would probably be easier on the eyes if you broke that up:

if (current_is_kswapd()) {
        if (!zone_reclaimable(zone))
                force_scan = true;
        else if (!mem_cgroup_online_from_lruvec(lruvec))
                force_scan = true;
} else if (!global_reclaim(sc)) {
                force_scan = true;
}

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