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Message-ID: <CAK1f24=8k9uG7yWB2R3a_xBOuNT8dg4GCRk5i=wpGGrCvLg+ow@mail.gmail.com>
Date: Thu, 18 Apr 2024 20:48:12 +0800
From: Lance Yang <ioworker0@...il.com>
To: David Hildenbrand <david@...hat.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 Thu, Apr 18, 2024 at 8:44 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 18.04.24 14:33, Lance Yang wrote:
> > On Thu, Apr 18, 2024 at 8:03 PM David Hildenbrand <david@...hatcom> 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;
Yep, I understood.
Thanks,
Lance
>
> --
> Cheers,
>
> David / dhildenb
>
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