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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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