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SubjectRe: [PATCH 2/2] mm: khugepaged: check MMF_DISABLE_THP ahead of iterating over vmas
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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@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.
>
> 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.

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
>>>
>>>

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