Re: [PATCH 2/2] mm: khugepaged: check MMF_DISABLE_THP ahead of iterating over vmas

[Yang Shi <shy828301@gmail.com>]

On Sun, Apr 4, 2021 at 8:33 AM <yanfei.xu@windriver.com> wrote:
>
> From: Yanfei Xu <yanfei.xu@windriver.com>
>
> We could check MMF_DISABLE_THP ahead of iterating over all of vma.
> Otherwise if some mm_struct contain a large number of vma, there will
> be amounts meaningless cpu cycles cost.
>
> BTW, drop an unnecessary cond_resched(), because there is a another
> cond_resched() followed it and no consumed invocation between them.
>
> Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
> ---
>  mm/khugepaged.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 2efe1d0c92ed..c293ec4a94ea 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2094,6 +2094,8 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
>          */
>         if (unlikely(!mmap_read_trylock(mm)))
>                 goto breakouterloop_mmap_lock;
> +       if (test_bit(MMF_DISABLE_THP, &mm->flags))
> +               goto breakouterloop_mmap_lock;

It is fine to check this flag. But mmap_lock has been acquired so you
should jump to breakouterloop.

>         if (likely(!khugepaged_test_exit(mm)))
>                 vma = find_vma(mm, khugepaged_scan.address);
>
> @@ -2101,7 +2103,6 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
>         for (; vma; vma = vma->vm_next) {
>                 unsigned long hstart, hend;
>
> -               cond_resched();

I don't have a strong opinion for removing this cond_resched(). But
IIUC khugepaged is a best effort job there is no harm to keep it IMHO.

>                 if (unlikely(khugepaged_test_exit(mm))) {
>                         progress++;
>                         break;
> --
> 2.27.0
>
>