Re: [PATCH 3/4] memcg: hierarchical reclaim by CSS ID

[Li Zefan]

>  /*
> - * Dance down the hierarchy if needed to reclaim memory. We remember the
> - * last child we reclaimed from, so that we don't end up penalizing
> - * one child extensively based on its position in the children list.
> + * Visit the first child (need not be the first child as per the ordering
> + * of the cgroup list, since we track last_scanned_child) of @mem and use
> + * that to reclaim free pages from.
> + */
> +static struct mem_cgroup *
> +mem_cgroup_select_victim(struct mem_cgroup *root_mem)
> +{
> +	struct mem_cgroup *ret = NULL;
> +	struct cgroup_subsys_state *css;
> +	int nextid, found;
> +
> +	if (!root_mem->use_hierarchy) {
> +		spin_lock(&root_mem->reclaim_param_lock);
> +		root_mem->scan_age++;
> +		spin_unlock(&root_mem->reclaim_param_lock);
> +		css_get(&root_mem->css);
> +		ret = root_mem;
> +	}
> +
> +	while (!ret) {
> +		rcu_read_lock();
> +		nextid = root_mem->last_scanned_child + 1;
> +		css = css_get_next(&mem_cgroup_subsys, nextid, &root_mem->css,
> +				   &found);
> +		if (css && css_is_populated(css) && css_tryget(css))

I don't see why you need to check css_is_populated(css) ?

> +			ret = container_of(css, struct mem_cgroup, css);
> +
> +		rcu_read_unlock();
> +		/* Updates scanning parameter */
> +		spin_lock(&root_mem->reclaim_param_lock);
> +		if (!css) {
> +			/* this means start scan from ID:1 */
> +			root_mem->last_scanned_child = 0;
> +			root_mem->scan_age++;
> +		} else
> +			root_mem->last_scanned_child = found;
> +		spin_unlock(&root_mem->reclaim_param_lock);
> +	}
> +
> +	return ret;
> +}
> +
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