Re: [PATCH v1] mm/contpte: Optimize loop to reduce redundant operations

[David Laight <david.laight.linux@gmail.com>]

On Fri, 11 Apr 2025 09:25:39 +1200
Barry Song <21cnbao@gmail.com> wrote:

> On Mon, Apr 7, 2025 at 9:23 PM Xavier <xavier_qy@163.com> wrote:
> >
> > This commit optimizes the contpte_ptep_get function by adding early
> >  termination logic. It checks if the dirty and young bits of orig_pte
> >  are already set and skips redundant bit-setting operations during
> >  the loop. This reduces unnecessary iterations and improves performance.
> >
> > Signed-off-by: Xavier <xavier_qy@163.com>
> > ---
> >  arch/arm64/mm/contpte.c | 13 +++++++++++--
> >  1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
> > index bcac4f55f9c1..ca15d8f52d14 100644
> > --- a/arch/arm64/mm/contpte.c
> > +++ b/arch/arm64/mm/contpte.c
> > @@ -163,17 +163,26 @@ pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
> >
> >         pte_t pte;
> >         int i;
> > +       bool dirty = false;
> > +       bool young = false;
> >
> >         ptep = contpte_align_down(ptep);
> >
> >         for (i = 0; i < CONT_PTES; i++, ptep++) {
> >                 pte = __ptep_get(ptep);
> >
> > -               if (pte_dirty(pte))
> > +               if (!dirty && pte_dirty(pte)) {
> > +                       dirty = true;
> >                         orig_pte = pte_mkdirty(orig_pte);
> > +               }
> >
> > -               if (pte_young(pte))
> > +               if (!young && pte_young(pte)) {
> > +                       young = true;
> >                         orig_pte = pte_mkyoung(orig_pte);
> > +               }
> > +
> > +               if (dirty && young)
> > +                       break;  
> 
> This kind of optimization is always tricky. Dev previously tried a similar
> approach to reduce the loop count, but it ended up causing performance
> degradation:
> https://lore.kernel.org/linux-mm/20240913091902.1160520-1-dev.jain@arm.com/
> 
> So we may need actual data to validate this idea.

You might win with 3 loops.
The first looks for both 'dirty' and 'young'.
If it finds only one it jumps to a different loop that continues
the search but only looks for the other flag.

	David

> 
> >         }
> >
> >         return orig_pte;
> > --
> > 2.34.1
> >  
> 
> Thanks
> Barry
>