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Date: Thu, 18 Apr 2024 14:44:34 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, ryan.roberts@....com, 21cnbao@...il.com,
mhocko@...e.com, fengwei.yin@...el.com, zokeefe@...gle.com,
shy828301@...il.com, xiehuan09@...il.com, wangkefeng.wang@...wei.com,
songmuchun@...edance.com, peterx@...hat.com, minchan@...nel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 4/4] mm/madvise: optimize lazyfreeing with mTHP in
madvise_free
On 18.04.24 14:33, Lance Yang wrote:
> On Thu, Apr 18, 2024 at 8:03 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 18.04.24 12:57, Lance Yang wrote:
>>> This patch optimizes lazyfreeing with PTE-mapped mTHP[1]
>>> (Inspired by David Hildenbrand[2]). We aim to avoid unnecessary folio
>>> splitting if the large folio is fully mapped within the target range.
>>>
>>> If a large folio is locked or shared, or if we fail to split it, we just
>>> leave it in place and advance to the next PTE in the range. But note that
>>> the behavior is changed; previously, any failure of this sort would cause
>>> the entire operation to give up. As large folios become more common,
>>> sticking to the old way could result in wasted opportunities.
>>>
>>> On an Intel I5 CPU, lazyfreeing a 1GiB VMA backed by PTE-mapped folios of
>>> the same size results in the following runtimes for madvise(MADV_FREE) in
>>> seconds (shorter is better):
>>>
>>> Folio Size | Old | New | Change
>>> ------------------------------------------
>>> 4KiB | 0.590251 | 0.590259 | 0%
>>> 16KiB | 2.990447 | 0.185655 | -94%
>>> 32KiB | 2.547831 | 0.104870 | -95%
>>> 64KiB | 2.457796 | 0.052812 | -97%
>>> 128KiB | 2.281034 | 0.032777 | -99%
>>> 256KiB | 2.230387 | 0.017496 | -99%
>>> 512KiB | 2.189106 | 0.010781 | -99%
>>> 1024KiB | 2.183949 | 0.007753 | -99%
>>> 2048KiB | 0.002799 | 0.002804 | 0%
>>>
>>> [1] https://lkml.kernel.org/r/20231207161211.2374093-5-ryan.roberts@arm.com
>>> [2] https://lore.kernel.org/linux-mm/20240214204435.167852-1-david@redhat.com
>>>
>>> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
>>> Signed-off-by: Lance Yang <ioworker0@...il.com>
>>> ---
>>> mm/madvise.c | 85 +++++++++++++++++++++++++++-------------------------
>>> 1 file changed, 44 insertions(+), 41 deletions(-)
>>>
>>> diff --git a/mm/madvise.c b/mm/madvise.c
>>> index 4597a3568e7e..375ab3234603 100644
>>> --- a/mm/madvise.c
>>> +++ b/mm/madvise.c
>>> @@ -643,6 +643,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>>> unsigned long end, struct mm_walk *walk)
>>>
>>> {
>>> + const cydp_t cydp_flags = CYDP_CLEAR_YOUNG | CYDP_CLEAR_DIRTY;
>>> struct mmu_gather *tlb = walk->private;
>>> struct mm_struct *mm = tlb->mm;
>>> struct vm_area_struct *vma = walk->vma;
>>> @@ -697,44 +698,57 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>>> continue;
>>>
>>> /*
>>> - * If pmd isn't transhuge but the folio is large and
>>> - * is owned by only this process, split it and
>>> - * deactivate all pages.
>>> + * If we encounter a large folio, only split it if it is not
>>> + * fully mapped within the range we are operating on. Otherwise
>>> + * leave it as is so that it can be marked as lazyfree. If we
>>> + * fail to split a folio, leave it in place and advance to the
>>> + * next pte in the range.
>>> */
>>> if (folio_test_large(folio)) {
>>> - int err;
>>> + bool any_young, any_dirty;
>>>
>>> - if (folio_likely_mapped_shared(folio))
>>> - break;
>>> - if (!folio_trylock(folio))
>>> - break;
>>> - folio_get(folio);
>>> - arch_leave_lazy_mmu_mode();
>>> - pte_unmap_unlock(start_pte, ptl);
>>> - start_pte = NULL;
>>> - err = split_folio(folio);
>>> - folio_unlock(folio);
>>> - folio_put(folio);
>>> - if (err)
>>> - break;
>>> - start_pte = pte =
>>> - pte_offset_map_lock(mm, pmd, addr, &ptl);
>>> - if (!start_pte)
>>> - break;
>>> - arch_enter_lazy_mmu_mode();
>>> - pte--;
>>> - addr -= PAGE_SIZE;
>>> - continue;
>>> + nr = madvise_folio_pte_batch(addr, end, folio, pte,
>>> + ptent, &any_young, NULL);
>>> +
>>> + if (nr < folio_nr_pages(folio)) {
>>> + int err;
>>> +
>>> + if (folio_likely_mapped_shared(folio))
>>> + continue;
>>> + if (!folio_trylock(folio))
>>> + continue;
>>> + folio_get(folio);
>>> + arch_leave_lazy_mmu_mode();
>>> + pte_unmap_unlock(start_pte, ptl);
>>> + start_pte = NULL;
>>> + err = split_folio(folio);
>>> + folio_unlock(folio);
>>> + folio_put(folio);
>>> + start_pte = pte =
>>> + pte_offset_map_lock(mm, pmd, addr, &ptl);
>>
>> I'd just put it on a single line.
>
> start_pte = pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
>
> I suddenly realized that putting it on a single line would exceed the
> 80-char limit.
Which is fine according to Documentation/process/coding-style.rst
.. as long as it aids readability.
Alternatively, the following might do:
pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
start_pte = pte;
--
Cheers,
David / dhildenb
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