Re: [PATCH v3 15/15] arm64/mm: Implement clear_ptes() to optimize exit()

Ryan Roberts
2023-12-12T12:02:27Z

On 08/12/2023 01:45, Alistair Popple wrote:
>
> Ryan Roberts <ryan.roberts@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 exit
>> performance 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 shows the results of running a kernel compilation workload
>> and measuring the cost of arm64_sys_exit_group() (which at ~1.5% is a
>> very small part of the overall workload).
>>
>> Benchmarks were run on Ampere Altra in 2 configs; single numa node and 2
>> numa nodes (tlbis are more expensive in 2 node config).
>>
>> - baseline: v6.7-rc1 + anonfolio-v7
>> - no-opt: contpte series without any attempt to optimize exit()
>> - simple-ptep_get_clear_full: simple optimization to exploit full=1.
>> ptep_get_clear_full() does not fully conform to its intended semantic
>> - robust-ptep_get_clear_full: similar to previous but
>> ptep_get_clear_full() fully conforms to its intended semantic
>> - clear_ptes: optimization implemented by this patch
>>
>> | config | numa=1 | numa=2 |
>> |----------------------------|--------|--------|
>> | baseline | 0% | 0% |
>> | no-opt | 190% | 768% |
>> | simple-ptep_get_clear_full | 8% | 29% |
>> | robust-ptep_get_clear_full | 21% | 19% |
>> | clear_ptes | 13% | 9% |
>>
>> In all cases, the cost of arm64_sys_exit_group() increases; this is
>> anticipated because there is more work to do to tear down the page
>> tables. But clear_ptes() gives the smallest increase overall.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>> arch/arm64/include/asm/pgtable.h | 32 ++++++++++++++++++++++++
>> arch/arm64/mm/contpte.c | 42 ++++++++++++++++++++++++++++++++
>> 2 files changed, 74 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index 9bd2f57a9e11..ff6b3cc9e819 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -1145,6 +1145,8 @@ 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);
>> 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,
>> @@ -1270,6 +1272,36 @@ 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, int full,
>> + unsigned int nr)
>> +{
>> + pte_t pte;
>> +
>> + if (!contpte_is_enabled(mm)) {
>
> I think it would be better to call the generic definition of
> clear_ptes() here. Obviously that won't exist if clear_ptes is defined
> here, but you could alcall it __clear_ptes() and #define clear_ptes
> __clear_ptes when the arch specific helper isn't defined.

My thinking was that we wouldn't bother to expose clear_ptes() when
CONFIG_ARM64_CONTPTE is disabled, and just fall back to the core-mm generic one.
But I think your proposal is probably cleaner and more consistent with
everything else. So I'll do this for the next version.

>
>> + unsigned int i;
>> + pte_t tail;
>> +
>> + 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);
>> + }
>> + } else 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);
>> +
>> + 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)
>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>> index 2a57df16bf58..34b43bde3fcd 100644
>> --- a/arch/arm64/mm/contpte.c
>> +++ b/arch/arm64/mm/contpte.c
>> @@ -257,6 +257,48 @@ 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)
>> +{
>> + /*
>> + * 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 (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)
>> {
>