Re: [RFC 3/3] mm: make swapin readahead to improve thp collapse rate

[Ebru Akagunduz <ebru.akagunduz@gmail.com>]

On Tue, Jun 16, 2015 at 11:20:20PM -0400, Rik van Riel wrote:
> On 06/16/2015 05:15 PM, Andrew Morton wrote:
> > On Sun, 14 Jun 2015 18:04:43 +0300 Ebru Akagunduz <ebru.akagunduz@gmail.com> wrote:
> > 
> >> This patch makes swapin readahead to improve thp collapse rate.
> >> When khugepaged scanned pages, there can be a few of the pages
> >> in swap area.
> >>
> >> With the patch THP can collapse 4kB pages into a THP when
> >> there are up to max_ptes_swap swap ptes in a 2MB range.
> >>
> >> The patch was tested with a test program that allocates
> >> 800MB of memory, writes to it, and then sleeps. I force
> >> the system to swap out all. Afterwards, the test program
> >> touches the area by writing, it skips a page in each
> >> 20 pages of the area.
> >>
> >> Without the patch, system did not swap in readahead.
> >> THP rate was %47 of the program of the memory, it
> >> did not change over time.
> >>
> >> With this patch, after 10 minutes of waiting khugepaged had
> >> collapsed %99 of the program's memory.
> >>
> >> ...
> >>
> >> +/*
> >> + * Bring missing pages in from swap, to complete THP collapse.
> >> + * Only done if khugepaged_scan_pmd believes it is worthwhile.
> >> + *
> >> + * Called and returns without pte mapped or spinlocks held,
> >> + * but with mmap_sem held to protect against vma changes.
> >> + */
> >> +
> >> +static void __collapse_huge_page_swapin(struct mm_struct *mm,
> >> +					struct vm_area_struct *vma,
> >> +					unsigned long address, pmd_t *pmd,
> >> +					pte_t *pte)
> >> +{
> >> +	unsigned long _address;
> >> +	pte_t pteval = *pte;
> >> +	int swap_pte = 0;
> >> +
> >> +	pte = pte_offset_map(pmd, address);
> >> +	for (_address = address; _address < address + HPAGE_PMD_NR*PAGE_SIZE;
> >> +	     pte++, _address += PAGE_SIZE) {
> >> +		pteval = *pte;
> >> +		if (is_swap_pte(pteval)) {
> >> +			swap_pte++;
> >> +			do_swap_page(mm, vma, _address, pte, pmd, 0x0, pteval);
> >> +			/* pte is unmapped now, we need to map it */
> >> +			pte = pte_offset_map(pmd, _address);
> >> +		}
> >> +	}
> >> +	pte--;
> >> +	pte_unmap(pte);
> >> +	trace_mm_collapse_huge_page_swapin(mm, vma->vm_start, swap_pte);
> >> +}
> > 
> > This is doing a series of synchronous reads.  That will be sloooow on
> > spinning disks.
> >
> > This function should be significantly faster if it first gets all the
> > necessary I/O underway.  I don't think we have a function which exactly
> > does this.  Perhaps generalise swapin_readahead() or open-code
> > something like
> 
> Looking at do_swap_page() and __lock_page_or_retry(), I guess
> there already is a way to do the above.
> 
> Passing a "flags" of FAULT_FLAG_ALLOW_RETRY|FAULT_FLAG_RETRY_NOWAIT
> to do_swap_page() should result in do_swap_page() returning with
> the pte unmapped and the mmap_sem still held if the page was not
> immediately available to map into the pte (trylock_page succeeds).
> 
> Ebru, can you try passing the above as the flags argument to
> do_swap_page(), and see what happens?

I will try and resent the patch series.

Thanks for suggestions.

Ebru

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