Re: [RFC v2 2/3] mm: make optimistic check for swapin readahead

Kirill A. Shutemov
2015-06-21T17:56:22Z

On Sat, Jun 20, 2015 at 02:28:05PM +0300, Ebru Akagunduz 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>
> ---
> Changes in v2:
> - Nothing changed
>
> 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;

I think this deserve sysfs knob. Like we have for
khugepaged_max_ptes_none.

> 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)) {

IIRC, is_swap_pte() is true for migration entries too,
Should we distinguish swap entries from migration entries here?

I guess no. On other hand we can expect migration entires to be converted
to normal ptes soon...

> + 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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--
Kirill A. Shutemov