Re: [RFC 2/3] mm: make optimistic check for swapin readahead

[Leon Romanovsky <leon@leon.nu>]

On Sun, Jun 14, 2015 at 6:04 PM, Ebru Akagunduz
<ebru.akagunduz@gmail.com> wrote:
> This patch makes optimistic check for swapin readahead
> to increase thp collapse rate. Before getting swapped
> out pages to memory, checks them and allows up to a
> certain number. It also prints out using tracepoints
> amount of unmapped ptes.
>
> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
> ---
>  include/trace/events/huge_memory.h | 11 +++++++----
>  mm/huge_memory.c                   | 13 ++++++++++---
>  2 files changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index 4b9049b..53c9f2e 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -9,9 +9,9 @@
>  TRACE_EVENT(mm_khugepaged_scan_pmd,
>
>         TP_PROTO(struct mm_struct *mm, unsigned long vm_start, bool writable,
> -               bool referenced, int none_or_zero, int collapse),
> +               bool referenced, int none_or_zero, int collapse, int unmapped),
>
> -       TP_ARGS(mm, vm_start, writable, referenced, none_or_zero, collapse),
> +       TP_ARGS(mm, vm_start, writable, referenced, none_or_zero, collapse, unmapped),
>
>         TP_STRUCT__entry(
>                 __field(struct mm_struct *, mm)
> @@ -20,6 +20,7 @@ TRACE_EVENT(mm_khugepaged_scan_pmd,
>                 __field(bool, referenced)
>                 __field(int, none_or_zero)
>                 __field(int, collapse)
> +               __field(int, unmapped)
>         ),
>
>         TP_fast_assign(
> @@ -29,15 +30,17 @@ TRACE_EVENT(mm_khugepaged_scan_pmd,
>                 __entry->referenced = referenced;
>                 __entry->none_or_zero = none_or_zero;
>                 __entry->collapse = collapse;
> +               __entry->unmapped = unmapped;
>         ),
>
> -       TP_printk("mm=%p, vm_start=%04lx, writable=%d, referenced=%d, none_or_zero=%d, collapse=%d",
> +       TP_printk("mm=%p, vm_start=%04lx, writable=%d, referenced=%d, none_or_zero=%d, collapse=%d, unmapped=%d",
>                 __entry->mm,
>                 __entry->vm_start,
>                 __entry->writable,
>                 __entry->referenced,
>                 __entry->none_or_zero,
> -               __entry->collapse)
> +               __entry->collapse,
> +               __entry->unmapped)
>  );
>
>  TRACE_EVENT(mm_collapse_huge_page,
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 9bb97fc..22bc0bf 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -24,6 +24,7 @@
>  #include <linux/migrate.h>
>  #include <linux/hashtable.h>
>  #include <linux/userfaultfd_k.h>
> +#include <linux/swapops.h>
>
>  #include <asm/tlb.h>
>  #include <asm/pgalloc.h>
> @@ -2639,11 +2640,11 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>  {
>         pmd_t *pmd;
>         pte_t *pte, *_pte;
> -       int ret = 0, none_or_zero = 0;
> +       int ret = 0, none_or_zero = 0, unmapped = 0;
>         struct page *page;
>         unsigned long _address;
>         spinlock_t *ptl;
> -       int node = NUMA_NO_NODE;
> +       int node = NUMA_NO_NODE, max_ptes_swap = HPAGE_PMD_NR/8;
Sorry for asking, my knoweldge of THP is very limited, but why did you
choose this default value?
>From the discussion followed by your patch
(https://lkml.org/lkml/2015/2/27/432), I got an impression that it is
not necessary right value.

>         bool writable = false, referenced = false;
>
>         VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> @@ -2657,6 +2658,12 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>         for (_address = address, _pte = pte; _pte < pte+HPAGE_PMD_NR;
>              _pte++, _address += PAGE_SIZE) {
>                 pte_t pteval = *_pte;
> +               if (is_swap_pte(pteval)) {
> +                       if (++unmapped <= max_ptes_swap)
> +                               continue;
> +                       else
> +                               goto out_unmap;
> +               }
>                 if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
>                         if (!userfaultfd_armed(vma) &&
>                             ++none_or_zero <= khugepaged_max_ptes_none)
> @@ -2701,7 +2708,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>  out_unmap:
>         pte_unmap_unlock(pte, ptl);
>         trace_mm_khugepaged_scan_pmd(mm, vma->vm_start, writable, referenced,
> -                                    none_or_zero, ret);
> +                                    none_or_zero, ret, unmapped);
>         if (ret) {
>                 node = khugepaged_find_target_node();
>                 /* collapse_huge_page will return with the mmap_sem released */
> --
> 1.9.1
>
> --
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-- 
Leon Romanovsky | Independent Linux Consultant
        www.leon.nu | leon@leon.nu

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