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Date: Mon, 10 Dec 2012 20:35:20 -0800
From: Ying Han <yinghan@...gle.com>
To: Michal Hocko <mhocko@...e.cz>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Johannes Weiner <hannes@...xchg.org>,
Tejun Heo <htejun@...il.com>,
Glauber Costa <glommer@...allels.com>,
Li Zefan <lizefan@...wei.com>
Subject: Re: [patch v2 4/6] memcg: simplify mem_cgroup_iter
On Mon, Nov 26, 2012 at 10:47 AM, Michal Hocko <mhocko@...e.cz> wrote:
> Current implementation of mem_cgroup_iter has to consider both css and
> memcg to find out whether no group has been found (css==NULL - aka the
> loop is completed) and that no memcg is associated with the found node
> (!memcg - aka css_tryget failed because the group is no longer alive).
> This leads to awkward tweaks like tests for css && !memcg to skip the
> current node.
>
> It will be much easier if we got rid off css variable altogether and
> only rely on memcg. In order to do that the iteration part has to skip
> dead nodes. This sounds natural to me and as a nice side effect we will
> get a simple invariant that memcg is always alive when non-NULL and all
> nodes have been visited otherwise.
>
> We could get rid of the surrounding while loop but keep it in for now to
> make review easier. It will go away in the following patch.
>
> Signed-off-by: Michal Hocko <mhocko@...e.cz>
> ---
> mm/memcontrol.c | 56 +++++++++++++++++++++++++++----------------------------
> 1 file changed, 27 insertions(+), 29 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6bcc97b..d1bc0e8 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1086,7 +1086,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> rcu_read_lock();
> while (!memcg) {
> struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
> - struct cgroup_subsys_state *css = NULL;
>
> if (reclaim) {
> int nid = zone_to_nid(reclaim->zone);
> @@ -1112,53 +1111,52 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
> * explicit visit.
> */
> if (!last_visited) {
> - css = &root->css;
> + memcg = root;
> } else {
> struct cgroup *prev_cgroup, *next_cgroup;
>
> prev_cgroup = (last_visited == root) ? NULL
> : last_visited->css.cgroup;
> - next_cgroup = cgroup_next_descendant_pre(prev_cgroup,
> - root->css.cgroup);
> - if (next_cgroup)
> - css = cgroup_subsys_state(next_cgroup,
> - mem_cgroup_subsys_id);
> - }
> +skip_node:
> + next_cgroup = cgroup_next_descendant_pre(
> + prev_cgroup, root->css.cgroup);
>
> - /*
> - * Even if we found a group we have to make sure it is alive.
> - * css && !memcg means that the groups should be skipped and
> - * we should continue the tree walk.
> - * last_visited css is safe to use because it is protected by
> - * css_get and the tree walk is rcu safe.
> - */
> - if (css == &root->css || (css && css_tryget(css)))
> - memcg = mem_cgroup_from_css(css);
> + /*
> + * Even if we found a group we have to make sure it is
> + * alive. css && !memcg means that the groups should be
> + * skipped and we should continue the tree walk.
> + * last_visited css is safe to use because it is
> + * protected by css_get and the tree walk is rcu safe.
> + */
> + if (next_cgroup) {
> + struct mem_cgroup *mem = mem_cgroup_from_cont(
> + next_cgroup);
> + if (css_tryget(&mem->css))
> + memcg = mem;
I see a functional change after this, where we now hold a refcnt of
css if memcg is root. It is not the case before this change.
--Ying
> + else {
> + prev_cgroup = next_cgroup;
> + goto skip_node;
> + }
> + }
> + }
>
> if (reclaim) {
> - struct mem_cgroup *curr = memcg;
> -
> if (last_visited)
> css_put(&last_visited->css);
>
> - if (css && !memcg)
> - curr = mem_cgroup_from_css(css);
> -
> /* make sure that the cached memcg is not removed */
> - if (curr)
> - css_get(&curr->css);
> - iter->last_visited = curr;
> + if (memcg)
> + css_get(&memcg->css);
> + iter->last_visited = memcg;
>
> - if (!css)
> + if (!memcg)
> iter->generation++;
> else if (!prev && memcg)
> reclaim->generation = iter->generation;
> spin_unlock(&iter->iter_lock);
> - } else if (css && !memcg) {
> - last_visited = mem_cgroup_from_css(css);
> }
>
> - if (prev && !css)
> + if (prev && !memcg)
> goto out_unlock;
> }
> out_unlock:
> --
> 1.7.10.4
>
--
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