Re: [PATCH] arm64: mm: drop tlb flush operation when clearing the access bit
From: Barry Song
Date: Thu Oct 26 2023 - 01:54:43 EST
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 :-)
Thanks
Barry