Re: [PATCH] mempolicy: Optimize queue_folios_pte_range by PTE batching

[Dev Jain]

On 16/04/25 1:03 pm, Baolin Wang wrote:
> On 2025/4/15 19:47, Dev Jain wrote:
> > On 15/04/25 3:47 pm, David Hildenbrand wrote:
> > > On 11.04.25 10:13, Dev Jain wrote:
> > > > After the check for queue_folio_required(), the code only cares 
> > > > about the
> > > > folio in the for loop, i.e the PTEs are redundant. Therefore, 
> > > > optimize this
> > > > loop by skipping over a PTE batch mapping the same folio.
> > > >
> > > > Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> > > > ---
> > > > Unfortunately I have only build tested this since my test 
> > > > environment is
> > > > broken.
> > > >
> > > >   mm/mempolicy.c | 12 +++++++++++-
> > > >   1 file changed, 11 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > > > index b28a1e6ae096..b019524da8a2 100644
> > > > --- a/mm/mempolicy.c
> > > > +++ b/mm/mempolicy.c
> > > > @@ -573,6 +573,9 @@ static int queue_folios_pte_range(pmd_t *pmd, 
> > > > unsigned long addr,
> > > >       pte_t *pte, *mapped_pte;
> > > >       pte_t ptent;
> > > >       spinlock_t *ptl;
> > > > +    int max_nr;
> > > > +    const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> > > > +    int nr = 1;
> > >
> > > Try sticking to reverse xmas tree, please. (not completely the case 
> > > here, but fpb_flags can easily be moved all he way to the top)
> >
> > I thought that the initializations were to be kept at the bottom.
> > Asking for future patches, should I put all declarations in reverse- 
> > xmas fashion (even those which I don't intend to touch w.r.t the patch 
> > logic), or do I do that for only my additions?
> >
> > > Also, why are you initializing nr to 1 here if you reinitialize it 
> > > below?
> >
> > Yup no need, I thought pte += nr will blow up due to nr not being 
> > initialized, but it won't because it gets executed just before the 
> > start of the second iteration.
> >
> > >  >       ptl = pmd_trans_huge_lock(pmd, vma);>       if (ptl) {
> > > > @@ -586,7 +589,8 @@ static int queue_folios_pte_range(pmd_t *pmd, 
> > > > unsigned long addr,
> > > >           walk->action = ACTION_AGAIN;
> > > >           return 0;
> > > >       }
> > >  > -    for (; addr != end; pte++, addr += PAGE_SIZE) {> +    for (; 
> > > addr != end; pte += nr, addr += nr * PAGE_SIZE) {
> > > > +        nr = 1;
> > > >           ptent = ptep_get(pte);
> > > >           if (pte_none(ptent))
> > > >               continue;
> > > > @@ -607,6 +611,11 @@ static int queue_folios_pte_range(pmd_t *pmd, 
> > > > unsigned long addr,
> > > >           if (!queue_folio_required(folio, qp))
> > > >               continue;
> > > >           if (folio_test_large(folio)) {
> > > > +            max_nr = (end - addr) >> PAGE_SHIFT;
> > > > +            if (max_nr != 1)
> > > > +                nr = folio_pte_batch(folio, addr, pte, ptent,
> > > > +                             max_nr, fpb_flags,
> > > > +                             NULL, NULL, NULL);
> > >
> > > We should probably do that immediately after we verified that 
> > > vm_normal_folio() have us something reasonable.
> >
> > But shouldn't we keep the small folio case separate to avoid the 
> > overhead of folio_pte_batch()?
> >
> > > >               /*
> > > >                * A large folio can only be isolated from LRU once,
> > > >                * but may be mapped by many PTEs (and Copy-On-Write may
> > > > @@ -633,6 +642,7 @@ static int queue_folios_pte_range(pmd_t *pmd, 
> > > > unsigned long addr,
> > > >               qp->nr_failed++;
> > > >               if (strictly_unmovable(flags))
> > > >                   break;
> > > > +            qp->nr_failed += nr - 1;
> > >
> > > Can't we do qp->nr_failed += nr; above?
> >
> > I did not dive deep into the significance of nr_failed, but I did that
> > to keep the code, before and after the change, equivalent:
> >
> > Claim: if we reach qp->nr_failed++ for a single pte, we will reach 
> > here for all ptes belonging to the same batch.
>
> Sorry, I missed the previous discussion (I replied to your new version). 
> I think this claim is incorrect, we will skip remaining ptes belonging 
> to the same batch with checking 'qp->large'.
>
>          if (folio_test_large(folio)) {
>              if (folio == qp->large)
>                  continue;
>              qp->large = folio;
>          }

Oops you are right, I missed that.