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    Subject[patch 06/10] mm: memcg: remove optimization of keeping the root_mem_cgroup LRU lists empty
    Date
    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. This is purely about the LRU
    list, root_mem_cgroup is still not charged.

    The overhead is temporary until the double-LRU scheme is going away
    completely.

    Signed-off-by: Johannes Weiner <jweiner@redhat.com>
    Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    Reviewed-by: Michal Hocko <mhocko@suse.cz>
    Reviewed-by: Kirill A. Shutemov <kirill@shutemov.name>
    ---
    mm/memcontrol.c | 12 ++----------
    1 files changed, 2 insertions(+), 10 deletions(-)

    diff --git a/mm/memcontrol.c b/mm/memcontrol.c
    index 21468e2..8b2af55 100644
    --- a/mm/memcontrol.c
    +++ b/mm/memcontrol.c
    @@ -958,8 +958,6 @@ 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);
    }
    @@ -984,13 +982,11 @@ 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]);
    }
    @@ -1004,13 +1000,11 @@ 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]);
    }
    @@ -1042,8 +1036,6 @@ 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]);
    }

    --
    1.7.6.2


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