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    SubjectRe: [PATCH v3 2/2] memcg: avoid THP split in task migration
    On Fri,  2 Mar 2012 15:13:09 -0500
    Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> wrote:

    > Currently we can't do task migration among memory cgroups without THP split,
    > which means processes heavily using THP experience large overhead in task
    > migration. This patch introduce the code for moving charge of THP and makes
    > THP more valuable.
    >
    > Changes from v2:
    > - add move_anon() and mapcount check
    >
    > Changes from v1:
    > - rename is_target_huge_pmd_for_mc() to is_target_thp_for_mc()
    > - remove pmd_present() check (it's buggy when pmd_trans_huge(pmd) is true)
    > - is_target_thp_for_mc() calls get_page() only when checks are passed
    > - unlock page table lock if !mc.precharge
    > - compare return value of is_target_thp_for_mc() explicitly to MC_TARGET_TYPE
    > - clean up &walk->mm->page_table_lock to &vma->vm_mm->page_table_lock
    > - add comment about why race with split_huge_page() does not happen
    >
    > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
    > Acked-by: Hillf Danton <dhillf@gmail.com>

    I write this after reading Andrew's one.


    > ---
    > mm/memcontrol.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++++++----
    > 1 files changed, 83 insertions(+), 6 deletions(-)
    >
    > diff --git linux-next-20120228.orig/mm/memcontrol.c linux-next-20120228/mm/memcontrol.c
    > index c83aeb5..b6d1bab 100644
    > --- linux-next-20120228.orig/mm/memcontrol.c
    > +++ linux-next-20120228/mm/memcontrol.c
    > @@ -5211,6 +5211,41 @@ static int is_target_pte_for_mc(struct vm_area_struct *vma,
    > return ret;
    > }
    >
    > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
    > +/*
    > + * We don't consider swapping or file mapped pages because THP does not
    > + * support them for now.
    > + * Caller should make sure that pmd_trans_huge(pmd) is true.
    > + */
    > +static int is_target_thp_for_mc(struct vm_area_struct *vma,
    > + unsigned long addr, pmd_t pmd, union mc_target *target)
    > +{
    > + struct page *page = NULL;
    > + struct page_cgroup *pc;
    > + int ret = 0;

    As Andrew pointed out, I agree MC_TARGET_NONE will be better.
    Maybe other part should be rewritten.

    > +
    > + page = pmd_page(pmd);
    > + VM_BUG_ON(!page || !PageHead(page));
    > + if (!move_anon() || page_mapcount(page) != 1)
    > + return 0;

    Could you add this ?
    ==
    static bool move_check_shared_map(struct page *page)
    {
    /*
    * Handling of shared pages between processes is a big trouble in memcg.
    * Now, we never move shared-mapped pages between memcg at 'task' moving because
    * we have no hint which task the page is really belongs to. For example,
    * When a task does "fork()-> move to the child other group -> exec()", the charges
    * should be stay in the original cgroup.
    * So, check mapcount to determine we can move or not.
    */
    return page_mapcount(page) != 1;
    }
    ==
    We may be able to support madvise(MOVE_MEMCG) or fadvise(MOVE_MEMCG), if necessary.



    > + pc = lookup_page_cgroup(page);
    > + if (PageCgroupUsed(pc) && pc->mem_cgroup == mc.from) {
    > + ret = MC_TARGET_PAGE;
    > + if (target) {
    > + get_page(page);
    > + target->page = page;

    Here, get_page() is used rather than get_page_unless_zero() because of
    __pmd_trans_huge_lock() is held ?



    > + }
    > + }
    > + return ret;
    > +}
    > +#else
    > +static inline int is_target_thp_for_mc(struct vm_area_struct *vma,
    > + unsigned long addr, pmd_t pmd, union mc_target *target)
    > +{
    > + return 0;
    > +}
    > +#endif
    > +
    > static int mem_cgroup_count_precharge_pte_range(pmd_t *pmd,
    > unsigned long addr, unsigned long end,
    > struct mm_walk *walk)
    > @@ -5219,7 +5254,14 @@ static int mem_cgroup_count_precharge_pte_range(pmd_t *pmd,
    > pte_t *pte;
    > spinlock_t *ptl;
    >
    > - split_huge_page_pmd(walk->mm, pmd);
    > + if (pmd_trans_huge_lock(pmd, vma) == 1) {
    > + if (is_target_thp_for_mc(vma, addr, *pmd, NULL)
    > + == MC_TARGET_PAGE)
    > + mc.precharge += HPAGE_PMD_NR;
    > + spin_unlock(&vma->vm_mm->page_table_lock);
    > + cond_resched();
    > + return 0;
    > + }

    Maybe hard to read ;) I think is_target_thp_for_mc includes too much '_'
    and short words...

    Hmm, how about renaming "is_target_thp_for_mc" as "pmd_move_target()" or some.
    (Ah yes, other handler's name should be fixed, too.)

    >
    > pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
    > for (; addr != end; pte++, addr += PAGE_SIZE)
    > @@ -5378,16 +5420,51 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,
    > struct vm_area_struct *vma = walk->private;
    > pte_t *pte;
    > spinlock_t *ptl;
    > + int type;
    > + union mc_target target;
    > + struct page *page;
    > + struct page_cgroup *pc;
    > +
    > + /*
    > + * We don't take compound_lock() here but no race with splitting thp
    > + * happens because:
    > + * - if pmd_trans_huge_lock() returns 1, the relevant thp is not
    > + * under splitting, which means there's no concurrent thp split,
    > + * - if another thread runs into split_huge_page() just after we
    > + * entered this if-block, the thread must wait for page table lock
    > + * to be unlocked in __split_huge_page_splitting(), where the main
    > + * part of thp split is not executed yet.
    > + */

    ok.


    > + if (pmd_trans_huge_lock(pmd, vma) == 1) {
    > + if (!mc.precharge) {
    > + spin_unlock(&vma->vm_mm->page_table_lock);
    > + cond_resched();

    Hm. Original code calls cond_resched() after 'scanning' the full pmd, 1024 entries.
    With THP, it just handles 1 entry. cond_resched() will not be required.

    > + return 0;
    > + }
    > + type = is_target_thp_for_mc(vma, addr, *pmd, &target);
    > + if (type == MC_TARGET_PAGE) {
    > + page = target.page;
    > + if (!isolate_lru_page(page)) {
    > + pc = lookup_page_cgroup(page);
    > + if (!mem_cgroup_move_account(page, HPAGE_PMD_NR,
    > + pc, mc.from, mc.to,
    > + false)) {
    > + mc.precharge -= HPAGE_PMD_NR;
    > + mc.moved_charge += HPAGE_PMD_NR;
    > + }
    > + putback_lru_page(page);
    > + }
    > + put_page(page);
    > + }
    > + spin_unlock(&vma->vm_mm->page_table_lock);
    > + cond_resched();

    ditto.


    > + return 0;
    > + }
    >
    > - split_huge_page_pmd(walk->mm, pmd);
    > retry:
    > pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
    > for (; addr != end; addr += PAGE_SIZE) {
    > pte_t ptent = *(pte++);
    > - union mc_target target;
    > - int type;
    > - struct page *page;
    > - struct page_cgroup *pc;
    > swp_entry_t ent;
    >
    > if (!mc.precharge)


    Thank you for your efforts!

    -Kame



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