linux-kernel - Re: [RFC PATCH v1 4/4] mm: page_io: Count successful mTHP zswap stores in vmstat.

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Message-ID: <CAGsJ_4yng2ES6C8OSA2qoW5AwQ+zNdEAYWcNpoXmOP+m84qprg@mail.gmail.com>
Date: Wed, 14 Aug 2024 19:53:19 +1200
From: Barry Song <21cnbao@...il.com>
To: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, hannes@...xchg.org, 
	yosryahmed@...gle.com, nphamcs@...il.com, ryan.roberts@....com, 
	ying.huang@...el.com, akpm@...ux-foundation.org, nanhai.zou@...el.com, 
	wajdi.k.feghali@...el.com, vinodh.gopal@...el.com
Subject: Re: [RFC PATCH v1 4/4] mm: page_io: Count successful mTHP zswap
 stores in vmstat.

On Wed, Aug 14, 2024 at 6:28 PM Kanchana P Sridhar
<kanchana.p.sridhar@...el.com> wrote:
>
> Added count_zswap_thp_swpout_vm_event() that will increment the
> appropriate mTHP/PMD vmstat event counters if zswap_store succeeds for
> a large folio:
>
> zswap_store mTHP order [0, HPAGE_PMD_ORDER-1] will increment these
> vmstat event counters:
>
>   ZSWPOUT_4KB_FOLIO
>   mTHP_ZSWPOUT_8kB
>   mTHP_ZSWPOUT_16kB
>   mTHP_ZSWPOUT_32kB
>   mTHP_ZSWPOUT_64kB
>   mTHP_ZSWPOUT_128kB
>   mTHP_ZSWPOUT_256kB
>   mTHP_ZSWPOUT_512kB
>   mTHP_ZSWPOUT_1024kB
>
> zswap_store of a PMD-size THP, i.e., mTHP order HPAGE_PMD_ORDER, will
> increment both these vmstat event counters:
>
>   ZSWPOUT_PMD_THP_FOLIO
>   mTHP_ZSWPOUT_2048kB
>
> Signed-off-by: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
> ---
>  mm/page_io.c | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 44 insertions(+)
>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index 0a150c240bf4..ab54d2060cc4 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -172,6 +172,49 @@ int generic_swapfile_activate(struct swap_info_struct *sis,
>         goto out;
>  }
>
> +/*
> + * Count vmstats for ZSWAP store of large folios (mTHP and PMD-size THP).
> + */
> +static inline void count_zswap_thp_swpout_vm_event(struct folio *folio)
> +{
> +       if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && folio_test_pmd_mappable(folio)) {
> +               count_vm_event(ZSWPOUT_PMD_THP_FOLIO);
> +               count_vm_event(mTHP_ZSWPOUT_2048kB);
> +       } else if (folio_order(folio) == 0) {
> +               count_vm_event(ZSWPOUT_4KB_FOLIO);
> +       } else if (IS_ENABLED(CONFIG_THP_SWAP)) {
> +               switch (folio_order(folio)) {
> +               case 1:
> +                       count_vm_event(mTHP_ZSWPOUT_8kB);
> +                       break;
> +               case 2:
> +                       count_vm_event(mTHP_ZSWPOUT_16kB);
> +                       break;
> +               case 3:
> +                       count_vm_event(mTHP_ZSWPOUT_32kB);
> +                       break;
> +               case 4:
> +                       count_vm_event(mTHP_ZSWPOUT_64kB);
> +                       break;
> +               case 5:
> +                       count_vm_event(mTHP_ZSWPOUT_128kB);
> +                       break;
> +               case 6:
> +                       count_vm_event(mTHP_ZSWPOUT_256kB);
> +                       break;
> +               case 7:
> +                       count_vm_event(mTHP_ZSWPOUT_512kB);
> +                       break;
> +               case 8:
> +                       count_vm_event(mTHP_ZSWPOUT_1024kB);
> +                       break;
> +               case 9:
> +                       count_vm_event(mTHP_ZSWPOUT_2048kB);
> +                       break;
> +               }

The number of orders is PMD_ORDER, also ilog2(MAX_PTRS_PER_PTE) .
PMD_ORDER isn't necessarily 9. It seems we need some general way
to handle this and avoid so many duplicated case 1, case 2.... case 9.

> +       }
> +}
> +
>  /*
>   * We may have stale swap cache pages in memory: notice
>   * them here and get rid of the unnecessary final write.
> @@ -196,6 +239,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
>                 return ret;
>         }
>         if (zswap_store(folio)) {
> +               count_zswap_thp_swpout_vm_event(folio);
>                 folio_start_writeback(folio);
>                 folio_unlock(folio);
>                 folio_end_writeback(folio);
> --
> 2.27.0
>

Thanks
Barry