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    SubjectRe: [patch 06/11] mm: memcg: remove optimization of keeping the root_mem_cgroup LRU lists empty
    On Mon 12-09-11 12:57:23, Johannes Weiner wrote:
    > root_mem_cgroup, lacking a configurable limit, was never subject to
    > limit reclaim, so the pages charged to it could be kept off its LRU
    > lists. They would be found on the global per-zone LRU lists upon
    > physical memory pressure and it made sense to avoid uselessly linking
    > them to both lists.
    >
    > The global per-zone LRU lists are about to go away on memcg-enabled
    > kernels, with all pages being exclusively linked to their respective
    > per-memcg LRU lists. As a result, pages of the root_mem_cgroup must
    > also be linked to its LRU lists again.

    Nevertheless we still do not charge them so this should be mentioned
    here?

    >
    > The overhead is temporary until the double-LRU scheme is going away
    > completely.
    >
    > Signed-off-by: Johannes Weiner <jweiner@redhat.com>

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

    > ---
    > mm/memcontrol.c | 12 ++----------
    > 1 files changed, 2 insertions(+), 10 deletions(-)
    >
    > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    > index 413e1f8..518f640 100644
    > --- a/mm/memcontrol.c
    > +++ b/mm/memcontrol.c
    > @@ -956,8 +956,6 @@ void mem_cgroup_del_lru_list(struct page *page, enum lru_list lru)
    > mz = page_cgroup_zoneinfo(pc->mem_cgroup, page);
    > /* huge page split is done under lru_lock. so, we have no races. */
    > MEM_CGROUP_ZSTAT(mz, lru) -= 1 << compound_order(page);
    > - if (mem_cgroup_is_root(pc->mem_cgroup))
    > - return;
    > VM_BUG_ON(list_empty(&pc->lru));
    > list_del_init(&pc->lru);
    > }
    > @@ -982,13 +980,11 @@ void mem_cgroup_rotate_reclaimable_page(struct page *page)
    > return;
    >
    > pc = lookup_page_cgroup(page);
    > - /* unused or root page is not rotated. */
    > + /* unused page is not rotated. */
    > if (!PageCgroupUsed(pc))
    > return;
    > /* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
    > smp_rmb();
    > - if (mem_cgroup_is_root(pc->mem_cgroup))
    > - return;
    > mz = page_cgroup_zoneinfo(pc->mem_cgroup, page);
    > list_move_tail(&pc->lru, &mz->lists[lru]);
    > }
    > @@ -1002,13 +998,11 @@ void mem_cgroup_rotate_lru_list(struct page *page, enum lru_list lru)
    > return;
    >
    > pc = lookup_page_cgroup(page);
    > - /* unused or root page is not rotated. */
    > + /* unused page is not rotated. */
    > if (!PageCgroupUsed(pc))
    > return;
    > /* Ensure pc->mem_cgroup is visible after reading PCG_USED. */
    > smp_rmb();
    > - if (mem_cgroup_is_root(pc->mem_cgroup))
    > - return;
    > mz = page_cgroup_zoneinfo(pc->mem_cgroup, page);
    > list_move(&pc->lru, &mz->lists[lru]);
    > }
    > @@ -1040,8 +1034,6 @@ void mem_cgroup_add_lru_list(struct page *page, enum lru_list lru)
    > /* huge page split is done under lru_lock. so, we have no races. */
    > MEM_CGROUP_ZSTAT(mz, lru) += 1 << compound_order(page);
    > SetPageCgroupAcctLRU(pc);
    > - if (mem_cgroup_is_root(pc->mem_cgroup))
    > - return;
    > list_add(&pc->lru, &mz->lists[lru]);
    > }
    >
    > --
    > 1.7.6
    >
    > --
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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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