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

[Dev Jain]

+others

On 11/04/25 2:55 am, Barry Song wrote:
> On Mon, Apr 7, 2025 at 9:23 PM Xavier <xavier_qy@xxxxxxx> 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@xxxxxxx>
> > ---
> >   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@xxxxxxx/
>
> So we may need actual data to validate this idea.

The original v2 patch does not work, I changed it to the following:

diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
index bcac4f55f9c1..db0ad38601db 100644
--- a/arch/arm64/mm/contpte.c
+++ b/arch/arm64/mm/contpte.c
@@ -152,6 +152,16 @@ void __contpte_try_unfold(struct mm_struct *mm, 
unsigned long addr,
 }
 EXPORT_SYMBOL_GPL(__contpte_try_unfold);

+#define CHECK_CONTPTE_FLAG(start, ptep, orig_pte, flag) \
+       int _start; \
+       pte_t *_ptep = ptep; \
+       for (_start = start; _start < CONT_PTES; _start++, ptep++) { \
+               if (pte_##flag(__ptep_get(_ptep))) { \
+                       orig_pte = pte_mk##flag(orig_pte); \
+                       break; \
+               } \
+       }
+
 pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
 {
        /*
@@ -169,11 +179,17 @@ pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte)
        for (i = 0; i < CONT_PTES; i++, ptep++) {
                pte = __ptep_get(ptep);

-               if (pte_dirty(pte))
+               if (pte_dirty(pte)) {
                        orig_pte = pte_mkdirty(orig_pte);
+                       CHECK_CONTPTE_FLAG(i, ptep, orig_pte, young);
+                       break;
+               }

-               if (pte_young(pte))
+               if (pte_young(pte)) {
                        orig_pte = pte_mkyoung(orig_pte);
+                       CHECK_CONTPTE_FLAG(i, ptep, orig_pte, dirty);
+                       break;
+               }
        }

        return orig_pte;

Some rudimentary testing with micromm reveals that this may be 
*slightly* faster. I cannot say for sure yet.

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