Date:   Tue, 19 Jul 2022 14:33:17 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Barry Song <21cnbao@...il.com>
Cc:     Anshuman Khandual <anshuman.khandual@....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

Barry Song <21cnbao@...il.com> writes:

> On Tue, Jul 19, 2022 at 5:47 PM Huang, Ying <ying.huang@...el.com> wrote:
>>
>> Barry Song <21cnbao@...il.com> writes:
>>
>> > 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);
>> >  }
>>
>> IS_ENABLED(CONFIG_THP_SWAP) doesn't match the name too.  It's an option
>> selected by users.  arch_thp_swp_supported() is to report the
>> capability.
>
> Hi Ying,
> CONFIG_THP_SWAP implicitly includes ARCH_WANTS_THP_SWAP. So it seems
> a bit odd to have still another arch_wants_thp_swap().
> if the name of arch_thp_swp_supported is not sensible to you, will
> thp_swp_supported()
> without arch_ make more sense? a similar example is,
>
> static inline bool gigantic_page_runtime_supported(void)
> {
>         return IS_ENABLED(CONFIG_ARCH_HAS_GIGANTIC_PAGE);
> }

Here, the capability of the architecture is reported.  But
CONFIG_THP_SWAP is a user option.

I'm OK with the function name "arch_thp_swp_supported()".  I just think
that we should implement the function in a way that is consistent with
the function name as much as possible.  That is, don't return true on
architectures that THP swap isn't supported in fact.

> Otherwise, can we just keep the code as is according to Anshuman's suggestion?

Although I still think my way is better, I will not force you to do
that.  If you don't think that is better, you can use your original
implementation.

Best Regards,
Huang, Ying

> Thanks
> Barry
>
> }
>
>
>
>>
>> Best Regards,
>> Huang, Ying
>>
>> >> 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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