[PATCH] arm64: mm: drop tlb flush operation when clearing the access bit
Anshuman Khandual
anshuman.khandual at arm.com
Wed Oct 25 23:01:59 PDT 2023
On 10/26/23 11:24, Barry Song wrote:
> On Thu, Oct 26, 2023 at 12:55 PM Anshuman Khandual
> <anshuman.khandual at arm.com> 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@gmail.com/
>>> Signed-off-by: Baolin Wang <baolin.wang at linux.alibaba.com>
>>> ---
>>> 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.
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