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

[Ryan Roberts]

On 08/12/2023 01:45, Alistair Popple wrote:
> 
> Ryan Roberts <ryan.roberts@xxxxxxx> 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@xxxxxxx>
>> ---
>>  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)
>>  {
>