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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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

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