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    SubjectRe: [patch 04/11] mm: memcg: per-priority per-zone hierarchy scan generations
    On Mon, 12 Sep 2011 12:57:21 +0200
    Johannes Weiner <jweiner@redhat.com> wrote:

    > Memory cgroup limit reclaim currently picks one memory cgroup out of
    > the target hierarchy, remembers it as the last scanned child, and
    > reclaims all zones in it with decreasing priority levels.
    >
    > The new hierarchy reclaim code will pick memory cgroups from the same
    > hierarchy concurrently from different zones and priority levels, it
    > becomes necessary that hierarchy roots not only remember the last
    > scanned child, but do so for each zone and priority level.
    >
    > Furthermore, detecting full hierarchy round-trips reliably will become
    > crucial, so instead of counting on one iterator site seeing a certain
    > memory cgroup twice, use a generation counter that is increased every
    > time the child with the highest ID has been visited.
    >
    > Signed-off-by: Johannes Weiner <jweiner@redhat.com>

    I cannot image how this works. could you illustrate more with easy example ?

    Thanks,
    -Kame

    > ---
    > mm/memcontrol.c | 60 +++++++++++++++++++++++++++++++++++++++---------------
    > 1 files changed, 43 insertions(+), 17 deletions(-)
    >
    > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > index 912c7c7..f4b404e 100644
    > --- a/mm/memcontrol.c
    > +++ b/mm/memcontrol.c
    > @@ -121,6 +121,11 @@ struct mem_cgroup_stat_cpu {
    > unsigned long targets[MEM_CGROUP_NTARGETS];
    > };
    >
    > +struct mem_cgroup_iter_state {
    > + int position;
    > + unsigned int generation;
    > +};
    > +
    > /*
    > * per-zone information in memory controller.
    > */
    > @@ -131,6 +136,8 @@ struct mem_cgroup_per_zone {
    > struct list_head lists[NR_LRU_LISTS];
    > unsigned long count[NR_LRU_LISTS];
    >
    > + struct mem_cgroup_iter_state iter_state[DEF_PRIORITY + 1];
    > +
    > struct zone_reclaim_stat reclaim_stat;
    > struct rb_node tree_node; /* RB tree node */
    > unsigned long long usage_in_excess;/* Set to the value by which */
    > @@ -231,11 +238,6 @@ struct mem_cgroup {
    > * per zone LRU lists.
    > */
    > struct mem_cgroup_lru_info info;
    > - /*
    > - * While reclaiming in a hierarchy, we cache the last child we
    > - * reclaimed from.
    > - */
    > - int last_scanned_child;
    > int last_scanned_node;
    > #if MAX_NUMNODES > 1
    > nodemask_t scan_nodes;
    > @@ -781,9 +783,15 @@ struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
    > return memcg;
    > }
    >
    > +struct mem_cgroup_iter {
    > + struct zone *zone;
    > + int priority;
    > + unsigned int generation;
    > +};
    > +
    > static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
    > struct mem_cgroup *prev,
    > - bool remember)
    > + struct mem_cgroup_iter *iter)
    > {
    > struct mem_cgroup *mem = NULL;
    > int id = 0;
    > @@ -791,7 +799,7 @@ static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
    > if (!root)
    > root = root_mem_cgroup;
    >
    > - if (prev && !remember)
    > + if (prev && !iter)
    > id = css_id(&prev->css);
    >
    > if (prev && prev != root)
    > @@ -804,10 +812,20 @@ static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
    > }
    >
    > while (!mem) {
    > + struct mem_cgroup_iter_state *uninitialized_var(is);
    > struct cgroup_subsys_state *css;
    >
    > - if (remember)
    > - id = root->last_scanned_child;
    > + if (iter) {
    > + int nid = zone_to_nid(iter->zone);
    > + int zid = zone_idx(iter->zone);
    > + struct mem_cgroup_per_zone *mz;
    > +
    > + mz = mem_cgroup_zoneinfo(root, nid, zid);
    > + is = &mz->iter_state[iter->priority];
    > + if (prev && iter->generation != is->generation)
    > + return NULL;
    > + id = is->position;
    > + }
    >
    > rcu_read_lock();
    > css = css_get_next(&mem_cgroup_subsys, id + 1, &root->css, &id);
    > @@ -818,8 +836,13 @@ static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
    > id = 0;
    > rcu_read_unlock();
    >
    > - if (remember)
    > - root->last_scanned_child = id;
    > + if (iter) {
    > + is->position = id;
    > + if (!css)
    > + is->generation++;
    > + else if (!prev && mem)
    > + iter->generation = is->generation;
    > + }
    >
    > if (prev && !css)
    > return NULL;
    > @@ -842,14 +865,14 @@ static void mem_cgroup_iter_break(struct mem_cgroup *root,
    > * be used for reference counting.
    > */
    > #define for_each_mem_cgroup_tree(iter, root) \
    > - for (iter = mem_cgroup_iter(root, NULL, false); \
    > + for (iter = mem_cgroup_iter(root, NULL, NULL); \
    > iter != NULL; \
    > - iter = mem_cgroup_iter(root, iter, false))
    > + iter = mem_cgroup_iter(root, iter, NULL))
    >
    > #define for_each_mem_cgroup(iter) \
    > - for (iter = mem_cgroup_iter(NULL, NULL, false); \
    > + for (iter = mem_cgroup_iter(NULL, NULL, NULL); \
    > iter != NULL; \
    > - iter = mem_cgroup_iter(NULL, iter, false))
    > + iter = mem_cgroup_iter(NULL, iter, NULL))
    >
    > static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
    > {
    > @@ -1619,6 +1642,10 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_memcg,
    > bool check_soft = reclaim_options & MEM_CGROUP_RECLAIM_SOFT;
    > unsigned long excess;
    > unsigned long nr_scanned;
    > + struct mem_cgroup_iter iter = {
    > + .zone = zone,
    > + .priority = 0,
    > + };
    >
    > excess = res_counter_soft_limit_excess(&root_memcg->res) >> PAGE_SHIFT;
    >
    > @@ -1627,7 +1654,7 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_memcg,
    > noswap = true;
    >
    > while (1) {
    > - victim = mem_cgroup_iter(root_memcg, victim, true);
    > + victim = mem_cgroup_iter(root_memcg, victim, &iter);
    > if (!victim) {
    > loop++;
    > /*
    > @@ -4878,7 +4905,6 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
    > res_counter_init(&memcg->res, NULL);
    > res_counter_init(&memcg->memsw, NULL);
    > }
    > - memcg->last_scanned_child = 0;
    > memcg->last_scanned_node = MAX_NUMNODES;
    > INIT_LIST_HEAD(&memcg->oom_notify);
    >
    > --
    > 1.7.6
    >
    > --
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