Re: [PATCH v1] mm/contpte: Optimize loop to reduce redundant operations
From: Dev Jain
Date: Mon Apr 14 2025 - 04:51:44 EST
On 14/04/25 1:36 pm, Ryan Roberts wrote:
On 12/04/2025 06:05, Lance Yang wrote:
On Sat, Apr 12, 2025 at 1:30 AM Dev Jain <dev.jain@xxxxxxx> wrote:
+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.
Yep, this change works as expected, IIUC.
However, I'm still wondering if the added complexity is worth it for
such a slight/negligible performance gain. That said, if we have
solid numbers/data to back it up, all doubts would disappear ;)
I agree with Barry; we need clear performance improvement numbers to consider
this type of optimization. I doubt there will be measurable improvement for 4K
and 64K base pages (because the the number of PTEs in a contpte block are 16 and
32 respectively). But 16K base pages may benefit given there are 128 PTEs in a
contpte block for that case.
Also FWIW, I'm struggling to understand CHECK_CONTPTE_FLAG(); Perhaps something
like this would suffice?
The idea of CHECK_CONTPTE_FLAG() is to check only for the other flag
until it's found; the diff below will still keep checking for both until
both are found.
----8<----
diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
index 55107d27d3f8..7787b116b339 100644
--- a/arch/arm64/mm/contpte.c
+++ b/arch/arm64/mm/contpte.c
@@ -169,11 +169,17 @@ 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);
+ if (pte_young(orig_pte))
+ break;
+ }
- if (pte_young(pte))
+ if (pte_young(pte)) {
orig_pte = pte_mkyoung(orig_pte);
+ if (pte_dirty(orig_pte))
+ break;
+ }
}
return orig_pte;
----8<----
Thanks,
Ryan
Thanks,
Lance
}
return orig_pte;
--
2.34.1
Thanks
Barry