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    SubjectRe: [patch 04/10] mm: memcg: per-priority per-zone hierarchy scan generations
    [linux-foundation.org seems to be down, so I have changed Andrew's
    address]

    On Thu 29-09-11 23:00:58, Johannes Weiner 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.
    >
    > Until now, we reclaimed memcgs like this:
    >
    > mem = mem_cgroup_iter(root)
    > for each priority level:
    > for each zone in zonelist:
    > reclaim(mem, zone)
    >
    > But subsequent patches will move the memcg iteration inside the loop
    > over the zones:
    >
    > for each priority level:
    > for each zone in zonelist:
    > mem = mem_cgroup_iter(root)
    > reclaim(mem, zone)
    >
    > And to keep with the original scan order - memcg -> priority -> zone -
    > the last scanned memcg has to be remembered per zone and per priority
    > level.
    >
    > Furthermore, global reclaim will be switched to the hierarchy walk as
    > well. Different from limit reclaim, which can just recheck the limit
    > after some reclaim progress, its target is to scan all memcgs for the
    > desired zone pages, proportional to the memcg size, and so reliably
    > detecting a full hierarchy round-trip will become crucial.
    >
    > Currently, the code relies on one reclaimer encountering the same
    > memcg twice, but that is error-prone with concurrent reclaimers.
    > Instead, use a generation counter that is increased every time the
    > child with the highest ID has been visited, so that reclaimers can
    > stop when the generation changes.

    Both naming and the patch description is much nicer. Thanks!

    >
    > Signed-off-by: Johannes Weiner <jweiner@redhat.com>
    > Reviewed-by: Kirill A. Shutemov <kirill@shutemov.name>

    Reviewed-by: Michal Hocko <mhocko@suse.cz>

    > ---
    > mm/memcontrol.c | 65 +++++++++++++++++++++++++++++++++++++++---------------
    > 1 files changed, 47 insertions(+), 18 deletions(-)
    >
    > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > index 0ba59f6..38d195d 100644
    > --- a/mm/memcontrol.c
    > +++ b/mm/memcontrol.c
    > @@ -121,6 +121,13 @@ struct mem_cgroup_stat_cpu {
    > unsigned long targets[MEM_CGROUP_NTARGETS];
    > };
    >
    > +struct mem_cgroup_reclaim_iter {
    > + /* css_id of the last scanned hierarchy member */
    > + int position;
    > + /* scan generation, increased every round-trip */
    > + unsigned int generation;
    > +};
    > +
    > /*
    > * per-zone information in memory controller.
    > */
    > @@ -131,6 +138,8 @@ struct mem_cgroup_per_zone {
    > struct list_head lists[NR_LRU_LISTS];
    > unsigned long count[NR_LRU_LISTS];
    >
    > + struct mem_cgroup_reclaim_iter reclaim_iter[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 +240,6 @@ 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 +785,16 @@ struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
    > return memcg;
    > }
    >
    > -static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
    > - struct mem_cgroup *prev,
    > - bool reclaim)
    > +struct mem_cgroup_reclaim_cookie {
    > + struct zone *zone;
    > + int priority;
    > + unsigned int generation;
    > +};
    > +
    > +static struct mem_cgroup *
    > +mem_cgroup_iter(struct mem_cgroup *root,
    > + struct mem_cgroup *prev,
    > + struct mem_cgroup_reclaim_cookie *reclaim)
    > {
    > struct mem_cgroup *mem = NULL;
    > int id = 0;
    > @@ -804,10 +815,20 @@ static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
    > }
    >
    > while (!mem) {
    > + struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
    > struct cgroup_subsys_state *css;
    >
    > - if (reclaim)
    > - id = root->last_scanned_child;
    > + if (reclaim) {
    > + int nid = zone_to_nid(reclaim->zone);
    > + int zid = zone_idx(reclaim->zone);
    > + struct mem_cgroup_per_zone *mz;
    > +
    > + mz = mem_cgroup_zoneinfo(root, nid, zid);
    > + iter = &mz->reclaim_iter[reclaim->priority];
    > + if (prev && reclaim->generation != iter->generation)
    > + return NULL;
    > + id = iter->position;
    > + }
    >
    > rcu_read_lock();
    > css = css_get_next(&mem_cgroup_subsys, id + 1, &root->css, &id);
    > @@ -818,8 +839,13 @@ static struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
    > id = 0;
    > rcu_read_unlock();
    >
    > - if (reclaim)
    > - root->last_scanned_child = id;
    > + if (reclaim) {
    > + iter->position = id;
    > + if (!css)
    > + iter->generation++;
    > + else if (!prev && mem)
    > + reclaim->generation = iter->generation;
    > + }
    >
    > if (prev && !css)
    > return NULL;
    > @@ -842,14 +868,14 @@ 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 +1645,10 @@ 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_reclaim_cookie reclaim = {
    > + .zone = zone,
    > + .priority = 0,
    > + };
    >
    > excess = res_counter_soft_limit_excess(&root_memcg->res) >> PAGE_SHIFT;
    >
    > @@ -1627,7 +1657,7 @@ 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, &reclaim);
    > if (!victim) {
    > loop++;
    > /*
    > @@ -4878,7 +4908,6 @@ 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);
    >
    > --
    > 1.7.6.2
    >
    > --
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    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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