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SubjectRe: [PATCH 3/4] memcg: hierarchical reclaim by CSS ID
On Fri, 16 Jan 2009 09:29:48 +0800
Li Zefan <lizf@cn.fujitsu.com> wrote:

> > /*
> > - * 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) ?
>

Main reason is for sanity. I don't like to hold css->refcnt of not populated css.
Second reason is for avoinding unnecessary calls to try_to_free_pages(),
it's heavy. I should also add mem->res.usage == 0 case for skipping but not yet.

THanks,
-Kame

> > + 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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