Date:   Tue, 19 Jul 2022 16:03:10 +1200
From:   Barry Song <21cnbao@...il.com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        Linux-MM <linux-mm@...ck.org>,
        Steven Price <steven.price@....com>,
        Will Deacon <will@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        郭健 <guojian@...o.com>,
        hanchuanhua <hanchuanhua@...o.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Hugh Dickins <hughd@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Minchan Kim <minchan@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Barry Song <v-songbaohua@...o.com>,
        张诗明(Simon Zhang) 
        <zhangshiming@...o.com>
Subject: Re: [RESEND PATCH v3] arm64: enable THP_SWAP for arm64

On Tue, Jul 19, 2022 at 3:59 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Tue, Jul 19, 2022 at 3:35 PM Anshuman Khandual
> <anshuman.khandual@....com> wrote:
> >
> >
> >
> > On 7/19/22 08:58, Huang, Ying wrote:
> > > Anshuman Khandual <anshuman.khandual@....com> writes:
> > >
> > >> On 7/19/22 06:53, Barry Song wrote:
> > >>> On Tue, Jul 19, 2022 at 12:44 PM Huang, Ying <ying.huang@...el.com> wrote:
> > >>>>
> > >>>> Barry Song <21cnbao@...il.com> writes:
> > >>>>
> > >>>>> From: Barry Song <v-songbaohua@...o.com>
> > >>>>>
> > >>>>> THP_SWAP has been proven to improve the swap throughput significantly
> > >>>>> on x86_64 according to commit bd4c82c22c367e ("mm, THP, swap: delay
> > >>>>> splitting THP after swapped out").
> > >>>>> As long as arm64 uses 4K page size, it is quite similar with x86_64
> > >>>>> by having 2MB PMD THP. THP_SWAP is architecture-independent, thus,
> > >>>>> enabling it on arm64 will benefit arm64 as well.
> > >>>>> A corner case is that MTE has an assumption that only base pages
> > >>>>> can be swapped. We won't enable THP_SWAP for ARM64 hardware with
> > >>>>> MTE support until MTE is reworked to coexist with THP_SWAP.
> > >>>>>
> > >>>>> A micro-benchmark is written to measure thp swapout throughput as
> > >>>>> below,
> > >>>>>
> > >>>>>  unsigned long long tv_to_ms(struct timeval tv)
> > >>>>>  {
> > >>>>>       return tv.tv_sec * 1000 + tv.tv_usec / 1000;
> > >>>>>  }
> > >>>>>
> > >>>>>  main()
> > >>>>>  {
> > >>>>>       struct timeval tv_b, tv_e;;
> > >>>>>  #define SIZE 400*1024*1024
> > >>>>>       volatile void *p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
> > >>>>>                               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> > >>>>>       if (!p) {
> > >>>>>               perror("fail to get memory");
> > >>>>>               exit(-1);
> > >>>>>       }
> > >>>>>
> > >>>>>       madvise(p, SIZE, MADV_HUGEPAGE);
> > >>>>>       memset(p, 0x11, SIZE); /* write to get mem */
> > >>>>>
> > >>>>>       gettimeofday(&tv_b, NULL);
> > >>>>>       madvise(p, SIZE, MADV_PAGEOUT);
> > >>>>>       gettimeofday(&tv_e, NULL);
> > >>>>>
> > >>>>>       printf("swp out bandwidth: %ld bytes/ms\n",
> > >>>>>                       SIZE/(tv_to_ms(tv_e) - tv_to_ms(tv_b)));
> > >>>>>  }
> > >>>>>
> > >>>>> Testing is done on rk3568 64bit quad core processor Quad Core
> > >>>>> Cortex-A55 platform - ROCK 3A.
> > >>>>> thp swp throughput w/o patch: 2734bytes/ms (mean of 10 tests)
> > >>>>> thp swp throughput w/  patch: 3331bytes/ms (mean of 10 tests)
> > >>>>>
> > >>>>> Cc: "Huang, Ying" <ying.huang@...el.com>
> > >>>>> Cc: Minchan Kim <minchan@...nel.org>
> > >>>>> Cc: Johannes Weiner <hannes@...xchg.org>
> > >>>>> Cc: Hugh Dickins <hughd@...gle.com>
> > >>>>> Cc: Andrea Arcangeli <aarcange@...hat.com>
> > >>>>> Cc: Anshuman Khandual <anshuman.khandual@....com>
> > >>>>> Cc: Steven Price <steven.price@....com>
> > >>>>> Cc: Yang Shi <shy828301@...il.com>
> > >>>>> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> > >>>>> ---
> > >>>>>  -v3:
> > >>>>>  * refine the commit log;
> > >>>>>  * add a benchmark result;
> > >>>>>  * refine the macro of arch_thp_swp_supported
> > >>>>>  Thanks to the comments of Anshuman, Andrew, Steven
> > >>>>>
> > >>>>>  arch/arm64/Kconfig               |  1 +
> > >>>>>  arch/arm64/include/asm/pgtable.h |  6 ++++++
> > >>>>>  include/linux/huge_mm.h          | 12 ++++++++++++
> > >>>>>  mm/swap_slots.c                  |  2 +-
> > >>>>>  4 files changed, 20 insertions(+), 1 deletion(-)
> > >>>>>
> > >>>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > >>>>> index 1652a9800ebe..e1c540e80eec 100644
> > >>>>> --- a/arch/arm64/Kconfig
> > >>>>> +++ b/arch/arm64/Kconfig
> > >>>>> @@ -101,6 +101,7 @@ config ARM64
> > >>>>>       select ARCH_WANT_HUGETLB_PAGE_OPTIMIZE_VMEMMAP
> > >>>>>       select ARCH_WANT_LD_ORPHAN_WARN
> > >>>>>       select ARCH_WANTS_NO_INSTR
> > >>>>> +     select ARCH_WANTS_THP_SWAP if ARM64_4K_PAGES
> > >>>>>       select ARCH_HAS_UBSAN_SANITIZE_ALL
> > >>>>>       select ARM_AMBA
> > >>>>>       select ARM_ARCH_TIMER
> > >>>>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > >>>>> index 0b6632f18364..78d6f6014bfb 100644
> > >>>>> --- a/arch/arm64/include/asm/pgtable.h
> > >>>>> +++ b/arch/arm64/include/asm/pgtable.h
> > >>>>> @@ -45,6 +45,12 @@
> > >>>>>       __flush_tlb_range(vma, addr, end, PUD_SIZE, false, 1)
> > >>>>>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> > >>>>>
> > >>>>> +static inline bool arch_thp_swp_supported(void)
> > >>>>> +{
> > >>>>> +     return !system_supports_mte();
> > >>>>> +}
> > >>>>> +#define arch_thp_swp_supported arch_thp_swp_supported
> > >>>>> +
> > >>>>>  /*
> > >>>>>   * Outside of a few very special situations (e.g. hibernation), we always
> > >>>>>   * use broadcast TLB invalidation instructions, therefore a spurious page
> > >>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > >>>>> index de29821231c9..4ddaf6ad73ef 100644
> > >>>>> --- a/include/linux/huge_mm.h
> > >>>>> +++ b/include/linux/huge_mm.h
> > >>>>> @@ -461,4 +461,16 @@ static inline int split_folio_to_list(struct folio *folio,
> > >>>>>       return split_huge_page_to_list(&folio->page, list);
> > >>>>>  }
> > >>>>>
> > >>>>> +/*
> > >>>>> + * archs that select ARCH_WANTS_THP_SWAP but don't support THP_SWP due to
> > >>>>> + * limitations in the implementation like arm64 MTE can override this to
> > >>>>> + * false
> > >>>>> + */
> > >>>>> +#ifndef arch_thp_swp_supported
> > >>>>> +static inline bool arch_thp_swp_supported(void)
> > >>>>> +{
> > >>>>> +     return true;
> > >>>>> +}
> > >>>>
> > >>>> How about the following?
> > >>>>
> > >>>> static inline bool arch_wants_thp_swap(void)
> > >>>> {
> > >>>>      return IS_ENABLED(ARCH_WANTS_THP_SWAP);
> > >>>> }
> > >>>
> > >>> This looks good. then i'll need to change arm64 to
> > >>>
> > >>>  +static inline bool arch_thp_swp_supported(void)
> > >>>  +{
> > >>>  +     return IS_ENABLED(ARCH_WANTS_THP_SWAP) &&  !system_supports_mte();
> > >>>  +}
> > >>
> > >> Why ? CONFIG_THP_SWAP depends on ARCH_WANTS_THP_SWAP. In folio_alloc_swap(),
> > >> IS_ENABLED(CONFIG_THP_SWAP) enabled, will also imply ARCH_WANTS_THP_SWAP too
> > >> is enabled. Hence checking for ARCH_WANTS_THP_SWAP again does not make sense
> > >> either in the generic fallback stub, or in arm64 platform override. Because
> > >> without ARCH_WANTS_THP_SWAP enabled, arch_thp_swp_supported() should never
> > >> be called in the first place.
> > >
> > > For the only caller now, the checking looks redundant.  But the original
> > > proposed implementation as follows,
> > >
> > > static inline bool arch_thp_swp_supported(void)
> > > {
> > >      return true;
> > > }
> > >
> > > will return true even on architectures that don't support/want THP swap.
> >
> > But the function will never be called on for those platforms.
> >
> > > That will confuse people too.
> >
> > I dont see how.
> >
> > >
> > > And the "redundant" checking has no run time overhead, because compiler
> > > will do the trick.
> > I understand that, but dont think this indirection is necessary.
>
> Hi Anshuman, Hi Ying,
> Thanks for the comments of both of you. Does the below look ok?
>
> generic,
>
>  static inline bool arch_wants_thp_swap(void)
>   {
>       return IS_ENABLED(CONFIG_THP_SWAP);
>  }
>

sorry, i actually meant arch_thp_swp_supported() but not
arch_wants_thp_swap() in generic code,

 static inline bool arch_thp_swp_supported(void)
 {
      return IS_ENABLED(CONFIG_THP_SWAP);
 }

> arm64,
>
> static inline bool arch_thp_swp_supported(void)
> {
>      return IS_ENABLED(CONFIG_THP_SWAP) &&  !system_supports_mte();
> }
>
> caller,
>
> folio_alloc_swap(struct folio *folio)
> {
>
>   if (folio_test_large(folio)) {
>    - if (IS_ENABLED(CONFIG_THP_SWAP))
>   + if (arch_thp_swp_supported())
>         get_swap_pages(1, &entry, folio_nr_pages(folio));
>        goto out;
>   }
>
> Thanks
> Barry

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