Re: [PATCH] arm64: mm: drop tlb flush operation when clearing the access bit

From: Robin Murphy
Date: Thu Oct 26 2023 - 08:30:18 EST

On 26/10/2023 7:01 am, Anshuman Khandual wrote:


On 10/26/23 11:24, Barry Song wrote:
On Thu, Oct 26, 2023 at 12:55 PM Anshuman Khandual
<anshuman.khandual@xxxxxxx> wrote:



On 10/24/23 18:26, Baolin Wang wrote:
Now ptep_clear_flush_young() is only called by folio_referenced() to
check if the folio was referenced, and now it will call a tlb flush on
ARM64 architecture. However the tlb flush can be expensive on ARM64
servers, especially for the systems with a large CPU numbers.

TLB flush would be expensive on *any* platform with large CPU numbers ?


Similar to the x86 architecture, below comments also apply equally to
ARM64 architecture. So we can drop the tlb flush operation in
ptep_clear_flush_young() on ARM64 architecture to improve the performance.
"
/* Clearing the accessed bit without a TLB flush
* doesn't cause data corruption. [ It could cause incorrect
* page aging and the (mistaken) reclaim of hot pages, but the
* chance of that should be relatively low. ]
*
* So as a performance optimization don't flush the TLB when
* clearing the accessed bit, it will eventually be flushed by
* a context switch or a VM operation anyway. [ In the rare
* event of it not getting flushed for a long time the delay
* shouldn't really matter because there's no real memory
* pressure for swapout to react to. ]
*/

If always true, this sounds generic enough for all platforms, why only
x86 and arm64 ?

"
Running the thpscale to show some obvious improvements for compaction
latency with this patch:
base patched
Amean fault-both-1 1093.19 ( 0.00%) 1084.57 * 0.79%*
Amean fault-both-3 2566.22 ( 0.00%) 2228.45 * 13.16%*
Amean fault-both-5 3591.22 ( 0.00%) 3146.73 * 12.38%*
Amean fault-both-7 4157.26 ( 0.00%) 4113.67 * 1.05%*
Amean fault-both-12 6184.79 ( 0.00%) 5218.70 * 15.62%*
Amean fault-both-18 9103.70 ( 0.00%) 7739.71 * 14.98%*
Amean fault-both-24 12341.73 ( 0.00%) 10684.23 * 13.43%*
Amean fault-both-30 15519.00 ( 0.00%) 13695.14 * 11.75%*
Amean fault-both-32 16189.15 ( 0.00%) 14365.73 * 11.26%*
base patched
Duration User 167.78 161.03
Duration System 1836.66 1673.01
Duration Elapsed 2074.58 2059.75

Could you please point to the test repo you are running ?


Barry Song submitted a similar patch [1] before, that replaces the
ptep_clear_flush_young_notify() with ptep_clear_young_notify() in
folio_referenced_one(). However, I'm not sure if removing the tlb flush
operation is applicable to every architecture in kernel, so dropping
the tlb flush for ARM64 seems a sensible change.

The reasoning provided here sounds generic when true, hence there seems
to be no justification to keep it limited just for arm64 and x86. Also
what about pmdp_clear_flush_young_notify() when THP is enabled. Should
that also not do a TLB flush after clearing access bit ? Although arm64
does not enable __HAVE_ARCH_PMDP_CLEAR_YOUNG_FLUSH, rather depends on
the generic pmdp_clear_flush_young() which also does a TLB flush via
flush_pmd_tlb_range() while clearing the access bit.


Note: I am okay for both approach, if someone can help to ensure that
all architectures do not need the tlb flush when clearing the accessed
bit, then I also think Barry's patch is better (hope Barry can resend
his patch).

This paragraph belongs after the '----' below and not part of the commit
message.


[1] https://lore.kernel.org/lkml/20220617070555.344368-1-21cnbao@xxxxxxxxx/
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
arch/arm64/include/asm/pgtable.h | 31 ++++++++++++++++---------------
1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 0bd18de9fd97..2979d796ba9d 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -905,21 +905,22 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
unsigned long address, pte_t *ptep)
{
- int young = ptep_test_and_clear_young(vma, address, ptep);
-
- if (young) {
- /*
- * We can elide the trailing DSB here since the worst that can
- * happen is that a CPU continues to use the young entry in its
- * TLB and we mistakenly reclaim the associated page. The
- * window for such an event is bounded by the next
- * context-switch, which provides a DSB to complete the TLB
- * invalidation.
- */
- flush_tlb_page_nosync(vma, address);
- }
-
- return young;
+ /*
+ * This comment is borrowed from x86, but applies equally to ARM64:
+ *
+ * Clearing the accessed bit without a TLB flush doesn't cause
+ * data corruption. [ It could cause incorrect page aging and
+ * the (mistaken) reclaim of hot pages, but the chance of that
+ * should be relatively low. ]
+ *
+ * So as a performance optimization don't flush the TLB when
+ * clearing the accessed bit, it will eventually be flushed by
+ * a context switch or a VM operation anyway. [ In the rare
+ * event of it not getting flushed for a long time the delay
+ * shouldn't really matter because there's no real memory
+ * pressure for swapout to react to. ]
+ */
+ return ptep_test_and_clear_young(vma, address, ptep);
}

#ifdef CONFIG_TRANSPARENT_HUGEPAGE

There are three distinct concerns here

1) What are the chances of this misleading existing hot page reclaim process
2) How secondary MMU such as SMMU adapt to change in mappings without a flush
3) Could this break the architecture rule requiring a TLB flush after access
bit clear on a page table entry

In terms of all of above concerns, though 2 is different, which is an
issue between
cpu and non-cpu,
i feel kernel has actually dropped tlb flush at least for mglru, there
is no flush in
lru_gen_look_around(),

static bool folio_referenced_one(struct folio *folio,
struct vm_area_struct *vma, unsigned long address, void *arg)
{
...

if (pvmw.pte) {
if (lru_gen_enabled() &&
pte_young(ptep_get(pvmw.pte))) {
lru_gen_look_around(&pvmw);
referenced++;
}

if (ptep_clear_flush_young_notify(vma, address,
pvmw.pte))
referenced++;
}

return true;
}

and so is in walk_pte_range() of vmscan. linux has been surviving with
all above concerns for a while, believing it or not :-)

Although the first two concerns could be worked upon in the SW, kernel surviving
after breaking arch rules explicitly is not a correct state to be in IMHO.

AFAICS it's not breaking any rules as such. Indeed the architecture requires a TLBI to ensure that the access flag update is observed by the CPU, but what's being proposed here is relaxing Linux's own expectations to say actually we don't mind if the CPU *doesn't* observe the update straight away.

Thanks,
Robin.