Date:   Wed, 20 Jul 2022 08:06:42 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Barry Song <21cnbao@...il.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 7/20/22 07:16, Barry Song wrote:
> On Tue, Jul 19, 2022 at 4:04 PM Anshuman Khandual
> <anshuman.khandual@....com> wrote:
>>
>>
>>
>> On 7/19/22 09:29, Barry Song 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);
>>>  }
>>>
>>> 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;
>>>   }
>>
>> Current proposal in this patch LGTM, I dont see any reason for these changes.
> 
> OK, thanks, Anshuman. Can I collect this as a Reviewed-by?

Yes please.

Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>

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