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Date: Thu, 8 Mar 2012 16:30:08 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc: linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
Hillf Danton <dhillf@...il.com>, linux-kernel@...r.kernel.org,
Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v3 2/2] memcg: avoid THP split in task migration
On Fri, 2 Mar 2012 15:13:09 -0500
Naoya Horiguchi <n-horiguchi@...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.
Some review input from Kame and Andrea would be good, please.
> 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;
This should be MC_TARGET_NONE. And this function should have a return
type of "enum mc_target_type". And local variable `ret' should have
type "enum mc_target_type" as well.
Also, the name "is_target_thp_for_mc" doesn't make sense: an "is_foo"
function should return a boolean result, but this function doesn't do
that.
> + page = pmd_page(pmd);
> + VM_BUG_ON(!page || !PageHead(page));
> + if (!move_anon() || page_mapcount(page) != 1)
More page_mapcount tricks, and we just fixed a bug in the other one and
Hugh got upset.
Can we please at least document what we're doing here? This reader
forgot, and cannot reremember.
> + return 0;
MC_TARGET_NONE.
> + 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;
> + }
> + }
> + 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;
MC_TARGET_NONE.
> +}
> +#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;
That code layout is rather an eyesore :(
This:
if (is_target_thp_for_mc(vma, addr, *pmd, NULL) == MC_TARGET_PAGE)
mc.precharge += HPAGE_PMD_NR;
is probably better, but still an eyesore. See if we can come up with a
shorter name than "is_target_thp_for_mc" and all will be fixed!
> + spin_unlock(&vma->vm_mm->page_table_lock);
> + cond_resched();
> + return 0;
> + }
>
> 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;
"enum mc_target_type". Also choose a more specific name? Perhaps
`target_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.
> + */
> + if (pmd_trans_huge_lock(pmd, vma) == 1) {
> + if (!mc.precharge) {
> + spin_unlock(&vma->vm_mm->page_table_lock);
> + cond_resched();
> + 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();
> + return 0;
> + }
cond_resched() is an ugly thing. Are we sure that it is needed here?
> - 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)
--
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