linux-kernel - Re: [PATCH v2] mm: mincore: use pte_batch

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Message-ID: <CAGsJ_4wKCpDi7ov0rx4C=rfi90nSGQrDx=4JyKgug4NXC4cYgQ@mail.gmail.com>
Date: Thu, 8 May 2025 19:42:58 +1200
From: Barry Song <21cnbao@...il.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ryan.roberts@....com, 
	dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: mincore: use pte_batch_bint() to batch process
 large folios

On Thu, May 8, 2025 at 4:09 PM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:
>
> When I tested the mincore() syscall, I observed that it takes longer with
> 64K mTHP enabled on my Arm64 server. The reason is the mincore_pte_range()
> still checks each PTE individually, even when the PTEs are contiguous,
> which is not efficient.
>
> Thus we can use pte_batch_hint() to get the batch number of the present
> contiguous PTEs, which can improve the performance. I tested the mincore()
> syscall with 1G anonymous memory populated with 64K mTHP, and observed an
> obvious performance improvement:
>
> w/o patch               w/ patch                changes
> 6022us                  549us                   +91%
>
> Moreover, I also tested mincore() with disabling mTHP/THP, and did not
> see any obvious regression for base pages.
>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
> Changes from v1:
>  - Change to use pte_batch_hint() to get the batch number, per Ryan.
>
> Note: I observed the min_t() can introduce a slight performance regression
> for base pages, so I change to add a batch size check for base pages,
> which can resolve the performance regression issue.
> ---
>  mm/mincore.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/mm/mincore.c b/mm/mincore.c
> index 832f29f46767..2e6a9123305e 100644
> --- a/mm/mincore.c
> +++ b/mm/mincore.c
> @@ -21,6 +21,7 @@
>
>  #include <linux/uaccess.h>
>  #include "swap.h"
> +#include "internal.h"
>
>  static int mincore_hugetlb(pte_t *pte, unsigned long hmask, unsigned long addr,
>                         unsigned long end, struct mm_walk *walk)
> @@ -105,6 +106,7 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>         pte_t *ptep;
>         unsigned char *vec = walk->private;
>         int nr = (end - addr) >> PAGE_SHIFT;
> +       int step, i;
>
>         ptl = pmd_trans_huge_lock(pmd, vma);
>         if (ptl) {
> @@ -118,16 +120,23 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>                 walk->action = ACTION_AGAIN;
>                 return 0;
>         }
> -       for (; addr != end; ptep++, addr += PAGE_SIZE) {
> +       for (; addr != end; ptep += step, addr += step * PAGE_SIZE) {
>                 pte_t pte = ptep_get(ptep);
>
> +               step = 1;
>                 /* We need to do cache lookup too for pte markers */
>                 if (pte_none_mostly(pte))
>                         __mincore_unmapped_range(addr, addr + PAGE_SIZE,
>                                                  vma, vec);
> -               else if (pte_present(pte))
> -                       *vec = 1;
> -               else { /* pte is a swap entry */
> +               else if (pte_present(pte)) {
> +                       unsigned int batch = pte_batch_hint(ptep, pte);
> +
> +                       if (batch > 1)
> +                               step = min_t(unsigned int, batch, nr);

Not quite sure if nr should be (end - addr) / PAGE_SIZE as nr
is always the initial value. For example, nr = 50, and we have
scanned 48 PTEs, then we have 2 ptes left. No?

> +
> +                       for (i = 0; i < step; i++)
> +                               vec[i] = 1;
> +               } else { /* pte is a swap entry */
>                         swp_entry_t entry = pte_to_swp_entry(pte);
>
>                         if (non_swap_entry(entry)) {
> @@ -146,7 +155,7 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>  #endif
>                         }
>                 }
> -               vec++;
> +               vec += step;
>         }
>         pte_unmap_unlock(ptep - 1, ptl);
>  out:
> --
> 2.43.5
>

Thanks
Barry