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Date: Wed, 29 Feb 2012 22:40:09 +0800
From: Hillf Danton <dhillf@...il.com>
To: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
linux-kernel@...r.kernel.org,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [RFC][PATCH] memcg: avoid THP split in task migration
On Wed, Feb 29, 2012 at 8:28 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> On Tue, 28 Feb 2012 16:12:32 -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.
>>
>> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
>> Cc: Hillf Danton <dhillf@...il.com>
>
>
> Thank you!
++hd;
>
> A comment below.
>
>> ---
>> mm/memcontrol.c | 76 ++++++++++++++++++++++++++++++++++++++++++++++++++----
>> 1 files changed, 70 insertions(+), 6 deletions(-)
>>
>> diff --git linux-next-20120228.orig/mm/memcontrol.c linux-next-20120228/mm/memcontrol.c
>> index c83aeb5..e97c041 100644
>> --- linux-next-20120228.orig/mm/memcontrol.c
>> +++ linux-next-20120228/mm/memcontrol.c
>> @@ -5211,6 +5211,42 @@ 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.
>> + */
>> +static int is_target_huge_pmd_for_mc(struct vm_area_struct *vma,
static int is_target_thp_for_mc(struct vm_area_struct *vma,
or
static int is_target_pmd_for_mc(struct vm_area_struct *vma,
sounds better?
>> + unsigned long addr, pmd_t pmd, union mc_target *target)
>> +{
>> + struct page *page = NULL;
>> + struct page_cgroup *pc;
>> + int ret = 0;
>> +
>> + if (pmd_present(pmd))
>> + page = pmd_page(pmd);
>> + if (!page)
>> + return 0;
>> + VM_BUG_ON(!PageHead(page));
With a huge and stable pmd, the above operations on page could be
compacted into one line?
page = pmd_page(pmd);
>> + get_page(page);
>> + pc = lookup_page_cgroup(page);
>> + if (PageCgroupUsed(pc) && pc->mem_cgroup == mc.from) {
>> + ret = MC_TARGET_PAGE;
>> + if (target)
After checking target, looks only get_page() needed?
>> + target->page = page;
>> + }
>> + if (!ret || !target)
>> + put_page(page);
>> + return ret;
>> +}
>> +#else
>> +static inline int is_target_huge_pmd_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 +5255,13 @@ 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_huge_pmd_for_mc(vma, addr, *pmd, NULL))
if (is_target_huge_pmd_for_mc(vma, addr, *pmd, NULL) == MC_TARGET_PAGE)
looks clearer
>> + mc.precharge += HPAGE_PMD_NR;
As HPAGE_PMD_NR is directly used, compiler beeps if THP disabled, I guess.
If yes, please cleanup huge_mm.h with s/BUG()/BUILD_BUG()/ and with
both HPAGE_PMD_ORDER and HPAGE_PMD_NR also defined,
to easy others a bit.
>> + spin_unlock(&walk->mm->page_table_lock);
spin_unlock(&vma->mm->page_table_lock);
looks clearer
>> + cond_resched();
>> + return 0;
>> + }
>>
>> pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>> for (; addr != end; pte++, addr += PAGE_SIZE)
>> @@ -5378,16 +5420,38 @@ 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;
>> +
>> + if (pmd_trans_huge_lock(pmd, vma) == 1) {
>> + if (!mc.precharge)
>> + return 0;
Bang, return without page table lock released.
>> + type = is_target_huge_pmd_for_mc(vma, addr, *pmd, &target);
>> + if (type == MC_TARGET_PAGE) {
>> + page = target.page;
>> + if (!isolate_lru_page(page)) {
>> + pc = lookup_page_cgroup(page);
>
> Here is a diffuclut point. Please see mem_cgroup_split_huge_fixup(). It splits
Hard and hard point IMO.
> updates memcg's status of splitted pages under lru_lock and compound_lock
> but not under mm->page_table_lock.
>
> Looking into split_huge_page()
>
> split_huge_page() # take anon_vma lock
> __split_huge_page()
> __split_huge_page_refcount() # take lru_lock, compound_lock.
> mem_cgroup_split_huge_fixup()
> __split_huge_page_map() # take page table lock.
>
[copied from Naoya-san's reply]
> I'm afraid this callchain is not correct.
s/correct/complete/
> Page table lock seems to be taken before we enter the main split work.
>
> split_huge_page
> take anon_vma lock
> __split_huge_page
> __split_huge_page_splitting
> lock page_table_lock <--- *1
> page_check_address_pmd
> unlock page_table_lock
Yeah, splitters are blocked.
Plus from the *ugly* documented lock function(another
cleanup needed), the embedded mmap_sem also blocks splitters.
That said, could we simply wait and see results of test cases?
-hd
/* mmap_sem must be held on entry */
static inline int pmd_trans_huge_lock(pmd_t *pmd,
struct vm_area_struct *vma)
{
VM_BUG_ON(!rwsem_is_locked(&vma->vm_mm->mmap_sem));
if (pmd_trans_huge(*pmd))
return __pmd_trans_huge_lock(pmd, vma);
else
return 0;
}
> __split_huge_page_refcount
> lock lru_lock
> compound_lock
> mem_cgroup_split_huge_fixup
> compound_unlock
> unlock lru_lock
> __split_huge_page_map
> lock page_table_lock
> ... some work
> unlock page_table_lock
> unlock anon_vma lock
>
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