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SubjectRe: [Experimental][PATCH 21/21] memcg-new-hierarchical-reclaim.patch
On Wed, 3 Dec 2008 14:14:23 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> Implement hierarchy reclaim by cgroup_id.
>
> What changes:
> - reclaim is not done by tree-walk algorithm
> - mem_cgroup->last_schan_child is ID, not pointer.
> - no cgroup_lock.
> - scanning order is just defined by ID's order.
> (Scan by round-robin logic.)
>
> Changelog: v1 -> v2
> - make use of css_tryget();
> - count # of loops rather than remembering position.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujisu.com>
>
>
> mm/memcontrol.c | 214 +++++++++++++++++++-------------------------------------
> 1 file changed, 75 insertions(+), 139 deletions(-)
>
(snip)
> /*
> - * 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.
> + * This routine select next memcg by ID. Using RCU and tryget().
> + * No cgroup_mutex is required.
> */
> static struct mem_cgroup *
> -mem_cgroup_get_first_node(struct mem_cgroup *root_mem)
> +mem_cgroup_select_victim(struct mem_cgroup *root_mem)
> {
> - struct cgroup *cgroup;
> + struct cgroup *cgroup, *root_cgroup;
> struct mem_cgroup *ret;
> - struct mem_cgroup *last_scan = root_mem->last_scanned_child;
> - bool obsolete = false;
> + int nextid, rootid, depth, found;
>
> - if (last_scan) {
> - if (css_under_removal(&last_scan->css))
> - obsolete = true;
> - } else
> - obsolete = true;
> + root_cgroup = root_mem->css.cgroup;
> + rootid = cgroup_id(root_cgroup);
> + depth = cgroup_depth(root_cgroup);
> + found = 0;
> + ret = NULL;
>
> - /*
> - * Scan all children under the mem_cgroup mem
> - */
> - cgroup_lock();
> - if (list_empty(&root_mem->css.cgroup->children)) {
> - ret = root_mem;
> - goto done;
> + rcu_read_lock();
> + 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);
> + goto out;
> }
>
I think you forgot "ret = root_mem".
I got NULL pointer dereference BUG in my test(I've not tested use_hierarchy case yet).


Thanks,
Daisuke Nishimura.

> - if (!root_mem->last_scanned_child || obsolete) {
> -
> - if (obsolete)
> - mem_cgroup_put(root_mem->last_scanned_child);
> -
> - cgroup = list_first_entry(&root_mem->css.cgroup->children,
> - struct cgroup, sibling);
> - ret = mem_cgroup_from_cont(cgroup);
> - mem_cgroup_get(ret);
> - } else
> - ret = mem_cgroup_get_next_node(root_mem->last_scanned_child,
> - root_mem);
> + while (!ret) {
> + /* ID:0 is not used by cgroup-id */
> + nextid = root_mem->last_scanned_child + 1;
> + cgroup = cgroup_get_next(nextid, rootid, depth, &found);
> + if (cgroup) {
> + spin_lock(&root_mem->reclaim_param_lock);
> + root_mem->last_scanned_child = found;
> + spin_unlock(&root_mem->reclaim_param_lock);
> + ret = mem_cgroup_from_cont(cgroup);
> + if (!css_tryget(&ret->css))
> + ret = NULL;
> + } else {
> + spin_lock(&root_mem->reclaim_param_lock);
> + root_mem->scan_age++;
> + root_mem->last_scanned_child = 0;
> + spin_unlock(&root_mem->reclaim_param_lock);
> + }
> + }
> +out:
> + rcu_read_unlock();
>
> -done:
> - root_mem->last_scanned_child = ret;
> - cgroup_unlock();
> return ret;
> }
>


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