[PATCH v4 16/16] arm64/mm: Implement clear_ptes() to optimize exit, munmap, dontneed
Ryan Roberts
ryan.roberts at arm.com
Wed Dec 20 00:42:49 PST 2023
On 20/12/2023 05:28, Alistair Popple wrote:
>
> Ryan Roberts <ryan.roberts at arm.com> writes:
>
>> With the core-mm changes in place to batch-clear ptes during
>> zap_pte_range(), we can take advantage of this in arm64 to greatly
>> reduce the number of tlbis we have to issue, and recover the lost
>> performance in exit, munmap and madvise(DONTNEED) incured when adding
>> support for transparent contiguous ptes.
>>
>> If we are clearing a whole contpte range, we can elide first unfolding
>> that range and save the tlbis. We just clear the whole range.
>>
>> The following microbenchmark results demonstate the effect of this
>> change on madvise(DONTNEED) performance for large pte-mapped folios.
>> madvise(dontneed) is called for each page of a 1G populated mapping and
>> the total time is measured. 100 iterations per run, 8 runs performed on
>> both Apple M2 (VM) and Ampere Altra (bare metal). Tests performed for
>> case where 1G memory is comprised of pte-mapped order-9 folios. Negative
>> is faster, positive is slower, compared to baseline upon which the
>> series is based:
>>
>> | dontneed | Apple M2 VM | Ampere Altra |
>> | order-9 |-------------------|-------------------|
>> | (pte-map) | mean | stdev | mean | stdev |
>> |---------------|---------|---------|---------|---------|
>> | baseline | 0.0% | 7.9% | 0.0% | 0.0% |
>> | before-change | -1.3% | 7.0% | 13.0% | 0.0% |
>> | after-change | -9.9% | 0.9% | 14.1% | 0.0% |
>>
>> The memory is initially all contpte-mapped and has to be unfolded (which
>> requires tlbi for the whole block) when the first page is touched (since
>> the test is madvise-ing 1 page at a time). Ampere Altra has high cost
>> for tlbi; this is why cost increases there.
>>
>> The following microbenchmark results demonstate the recovery (and
>> overall improvement) of munmap performance for large pte-mapped folios.
>> munmap is called for a 1G populated mapping and the function runtime is
>> measured. 100 iterations per run, 8 runs performed on both Apple M2 (VM)
>> and Ampere Altra (bare metal). Tests performed for case where 1G memory
>> is comprised of pte-mapped order-9 folios. Negative is faster, positive
>> is slower, compared to baseline upon which the series is based:
>>
>> | munmap | Apple M2 VM | Ampere Altra |
>> | order-9 |-------------------|-------------------|
>> | (pte-map) | mean | stdev | mean | stdev |
>> |---------------|---------|---------|---------|---------|
>> | baseline | 0.0% | 6.4% | 0.0% | 0.1% |
>> | before-change | 43.3% | 1.9% | 375.2% | 0.0% |
>> | after-change | -6.0% | 1.4% | -0.6% | 0.2% |
>>
>> Tested-by: John Hubbard <jhubbard at nvidia.com>
>> Signed-off-by: Ryan Roberts <ryan.roberts at arm.com>
>> ---
>> arch/arm64/include/asm/pgtable.h | 42 +++++++++++++++++++++++++++++
>> arch/arm64/mm/contpte.c | 45 ++++++++++++++++++++++++++++++++
>> 2 files changed, 87 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index d4805f73b9db..f5bf059291c3 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -953,6 +953,29 @@ static inline pte_t __ptep_get_and_clear(struct mm_struct *mm,
>> return pte;
>> }
>>
>> +static inline pte_t __clear_ptes(struct mm_struct *mm,
>> + unsigned long address, pte_t *ptep,
>> + unsigned int nr, int full)
>
> Ping on my previous comment - why not just use the generic version
> defined in patch 3 which is basically identical to this?
Perhaps I misunderstood your original comment - I thought this was what you were
suggesting - i.e. move this code out of the arm64 clear_ptes() impl into its own
__clear_ptes() helper, and always define an arm64 clear_ptes(), even when
ARM64_CONTPTE is not enabled.
I can use (and was in v3) the generic version when ARM64_CONTPTE is disabled.
But I can't use it when its enabled, because then arm64 needs its own
implementation to manage the contpte bit. And once it defines it's own version,
by defining the macro clear_ptes(), then the generic version is no longer
defined so I can't call it as part of this implementation. Even if I could, that
would be recursive.
Or perhaps I'm still not understanding your suggestion?
>
>> +{
>> + pte_t orig_pte = __ptep_get_and_clear(mm, address, ptep);
>> + unsigned int i;
>> + pte_t pte;
>> +
>> + for (i = 1; i < nr; i++) {
>> + address += PAGE_SIZE;
>> + ptep++;
>> + pte = __ptep_get_and_clear(mm, address, ptep);
>> +
>> + if (pte_dirty(pte))
>> + orig_pte = pte_mkdirty(orig_pte);
>> +
>> + if (pte_young(pte))
>> + orig_pte = pte_mkyoung(orig_pte);
>> + }
>> +
>> + return orig_pte;
>> +}
>> +
>> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> #define __HAVE_ARCH_PMDP_HUGE_GET_AND_CLEAR
>> static inline pmd_t pmdp_huge_get_and_clear(struct mm_struct *mm,
>> @@ -1151,6 +1174,8 @@ extern pte_t contpte_ptep_get(pte_t *ptep, pte_t orig_pte);
>> extern pte_t contpte_ptep_get_lockless(pte_t *orig_ptep);
>> extern void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
>> pte_t *ptep, pte_t pte, unsigned int nr);
>> +extern pte_t contpte_clear_ptes(struct mm_struct *mm, unsigned long addr,
>> + pte_t *ptep, unsigned int nr, int full);
>> extern int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
>> unsigned long addr, pte_t *ptep);
>> extern int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
>> @@ -1279,6 +1304,22 @@ static inline void pte_clear(struct mm_struct *mm,
>> __pte_clear(mm, addr, ptep);
>> }
>>
>> +#define clear_ptes clear_ptes
>> +static inline pte_t clear_ptes(struct mm_struct *mm,
>> + unsigned long addr, pte_t *ptep,
>> + unsigned int nr, int full)
>> +{
>> + pte_t pte;
>> +
>> + if (nr == 1) {
>> + contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
>> + pte = __ptep_get_and_clear(mm, addr, ptep);
>> + } else
>> + pte = contpte_clear_ptes(mm, addr, ptep, nr, full);
>> +
>> + return pte;
>> +}
>> +
>> #define __HAVE_ARCH_PTEP_GET_AND_CLEAR
>> static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
>> unsigned long addr, pte_t *ptep)
>> @@ -1366,6 +1407,7 @@ static inline int ptep_set_access_flags(struct vm_area_struct *vma,
>> #define set_pte __set_pte
>> #define set_ptes __set_ptes
>> #define pte_clear __pte_clear
>> +#define clear_ptes __clear_ptes
>> #define __HAVE_ARCH_PTEP_GET_AND_CLEAR
>> #define ptep_get_and_clear __ptep_get_and_clear
>> #define __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>> index 72e672024785..6f2a15ac5163 100644
>> --- a/arch/arm64/mm/contpte.c
>> +++ b/arch/arm64/mm/contpte.c
>> @@ -293,6 +293,51 @@ void contpte_set_ptes(struct mm_struct *mm, unsigned long addr,
>> }
>> EXPORT_SYMBOL(contpte_set_ptes);
>>
>> +pte_t contpte_clear_ptes(struct mm_struct *mm, unsigned long addr, pte_t *ptep,
>> + unsigned int nr, int full)
>> +{
>> + /*
>> + * If we cover a partial contpte block at the beginning or end of the
>> + * batch, unfold if currently folded. This makes it safe to clear some
>> + * of the entries while keeping others. contpte blocks in the middle of
>> + * the range, which are fully covered don't need to be unfolded because
>> + * we will clear the full block.
>> + */
>> +
>> + unsigned int i;
>> + pte_t pte;
>> + pte_t tail;
>> +
>> + if (!mm_is_user(mm))
>> + return __clear_ptes(mm, addr, ptep, nr, full);
>> +
>> + if (ptep != contpte_align_down(ptep) || nr < CONT_PTES)
>> + contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
>> +
>> + if (ptep + nr != contpte_align_down(ptep + nr))
>> + contpte_try_unfold(mm, addr + PAGE_SIZE * (nr - 1),
>> + ptep + nr - 1,
>> + __ptep_get(ptep + nr - 1));
>> +
>> + pte = __ptep_get_and_clear(mm, addr, ptep);
>> +
>> + for (i = 1; i < nr; i++) {
>> + addr += PAGE_SIZE;
>> + ptep++;
>> +
>> + tail = __ptep_get_and_clear(mm, addr, ptep);
>> +
>> + if (pte_dirty(tail))
>> + pte = pte_mkdirty(pte);
>> +
>> + if (pte_young(tail))
>> + pte = pte_mkyoung(pte);
>> + }
>> +
>> + return pte;
>> +}
>> +EXPORT_SYMBOL(contpte_clear_ptes);
>> +
>> int contpte_ptep_test_and_clear_young(struct vm_area_struct *vma,
>> unsigned long addr, pte_t *ptep)
>> {
>
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