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

[Yang Shi]

On Mon, Apr 5, 2021 at 8:05 PM Xu, Yanfei <yanfei.xu@xxxxxxxxxxxxx> wrote:
>
>
>
> On 4/6/21 10:51 AM, Xu, Yanfei wrote:
> >
> >
> > On 4/6/21 2:20 AM, Yang Shi wrote:
> >> [Please note: This e-mail is from an EXTERNAL e-mail address]
> >>
> >> On Sun, Apr 4, 2021 at 8:33 AM <yanfei.xu@xxxxxxxxxxxxx> wrote:
> >>>
> >>> From: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
> >>>
> >>> 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@xxxxxxxxxxxxx>
> >>> ---
> >>>   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.
> >
> > Oops! It's my fault. Thank you for pointing out this.
> > Will fix it in v2.
> >
> >>
> >>>          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.
> >>
> >
> > Yes, keeping it is no harm. But I think we should add it when we need.
> > Look at the blow codes, there are only some simple check between these
> > two cond_resched().  And we still have some cond_resched() in the
> > khugepaged_scan_file() and khugepaged_scan_pmd() which is the actual
> > wrok about collapsing. So I think it is unnecessary.  :)
> >
>
> BTW, the original author add this cond_resched() might be worry about
> the hugepage_vma_check() always return false due to the MMF_DISABLE_THP.
> But now we have moved it out of the for loop of iterating vma.

A little bit of archeology showed the cond_resched() was there in the
first place even before MMF_DISABLE_THP was introduced.

>
> um.. That is my guess..
>
> Thanks,
> Yanfei
>
> >          for (; vma; vma = vma->vm_next) {
> >                  unsigned long hstart, hend;
> >
> >          cond_resched();                 //here
> >                  if (unlikely(khugepaged_test_exit(mm))) {
> >                          progress++;
> >                          break;
> >                  }
> >                  if (!hugepage_vma_check(vma, vma->vm_flags)) {
> > skip:
> >                          progress++;
> >                          continue;
> >                  }
> >                  hstart = ALIGN(vma->vm_start, HPAGE_PMD_SIZE);
> >                  hend = ALIGN_DOWN(vma->vm_end, HPAGE_PMD_SIZE);
> >                  if (hstart >= hend)
> >                          goto skip;
> >                  if (khugepaged_scan.address > hend)
> >                          goto skip;
> >                  if (khugepaged_scan.address < hstart)
> >                          khugepaged_scan.address = hstart;
> >                  VM_BUG_ON(!IS_ALIGNED(khugepaged_scan.address,
> > HPAGE_PMD_SIZE));
> >
> >                  if (shmem_file(vma->vm_file) && !shmem_huge_enabled(vma))
> >                          goto skip;
> >
> >                  while (khugepaged_scan.address < hend) {
> >                          int ret;
> >                          cond_resched();        //here
> >
> >
> >>>                  if (unlikely(khugepaged_test_exit(mm))) {
> >>>                          progress++;
> >>>                          break;
> >>> --
> >>> 2.27.0
> >>>
> >>>