Re: [PATCH v3 14/15] arm64/mm: Implement ptep_set_wrprotects() to optimize fork()

[Ryan Roberts]

On 08/12/2023 01:37, Alistair Popple wrote:
> 
> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
> 
>> With the core-mm changes in place to batch-copy ptes during fork, we can
>> take advantage of this in arm64 to greatly reduce the number of tlbis we
>> have to issue, and recover the lost fork performance incured when adding
>> support for transparent contiguous ptes.
>>
>> If we are write-protecting a whole contig range, we can apply the
>> write-protection to the whole range and know that it won't change
>> whether the range should have the contiguous bit set or not. For ranges
>> smaller than the contig range, we will still have to unfold, apply the
>> write-protection, then fold if the change now means the range is
>> foldable.
>>
>> This optimization is possible thanks to the tightening of the Arm ARM in
>> respect to the definition and behaviour when 'Misprogramming the
>> Contiguous bit'. See section D21194 at
>> https://developer.arm.com/documentation/102105/latest/
>>
>> Performance tested with the following test written for the will-it-scale
>> framework:
>>
>> -------
>>
>> char *testcase_description = "fork and exit";
>>
>> void testcase(unsigned long long *iterations, unsigned long nr)
>> {
>> 	int pid;
>> 	char *mem;
>>
>> 	mem = malloc(SZ_128M);
>> 	assert(mem);
>> 	memset(mem, 1, SZ_128M);
>>
>> 	while (1) {
>> 		pid = fork();
>> 		assert(pid >= 0);
>>
>> 		if (!pid)
>> 			exit(0);
>>
>> 		waitpid(pid, NULL, 0);
>>
>> 		(*iterations)++;
>> 	}
>> }
>>
>> -------
>>
>> I see huge performance regression when PTE_CONT support was added, then
>> the regression is mostly fixed with the addition of this change. The
>> following shows regression relative to before PTE_CONT was enabled
>> (bigger negative value is bigger regression):
>>
>> |   cpus |   before opt |   after opt |
>> |-------:|-------------:|------------:|
>> |      1 |       -10.4% |       -5.2% |
>> |      8 |       -15.4% |       -3.5% |
>> |     16 |       -38.7% |       -3.7% |
>> |     24 |       -57.0% |       -4.4% |
>> |     32 |       -65.8% |       -5.4% |
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> ---
>>  arch/arm64/include/asm/pgtable.h | 30 ++++++++++++++++++++---
>>  arch/arm64/mm/contpte.c          | 42 ++++++++++++++++++++++++++++++++
>>  2 files changed, 69 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index 15bc9cf1eef4..9bd2f57a9e11 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -984,6 +984,16 @@ static inline void __ptep_set_wrprotect(struct mm_struct *mm,
>>  	} while (pte_val(pte) != pte_val(old_pte));
>>  }
>>  
>> +static inline void __ptep_set_wrprotects(struct mm_struct *mm,
>> +					unsigned long address, pte_t *ptep,
>> +					unsigned int nr)
>> +{
>> +	unsigned int i;
>> +
>> +	for (i = 0; i < nr; i++, address += PAGE_SIZE, ptep++)
>> +		__ptep_set_wrprotect(mm, address, ptep);
>> +}
>> +
>>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>  #define __HAVE_ARCH_PMDP_SET_WRPROTECT
>>  static inline void pmdp_set_wrprotect(struct mm_struct *mm,
>> @@ -1139,6 +1149,8 @@ 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,
>>  				unsigned long addr, pte_t *ptep);
>> +extern void contpte_set_wrprotects(struct mm_struct *mm, unsigned long addr,
>> +				pte_t *ptep, unsigned int nr);
>>  extern int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
>>  				unsigned long addr, pte_t *ptep,
>>  				pte_t entry, int dirty);
>> @@ -1290,13 +1302,25 @@ static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
>>  	return contpte_ptep_clear_flush_young(vma, addr, ptep);
>>  }
>>  
>> +#define ptep_set_wrprotects ptep_set_wrprotects
>> +static inline void ptep_set_wrprotects(struct mm_struct *mm, unsigned long addr,
>> +				pte_t *ptep, unsigned int nr)
>> +{
>> +	if (!contpte_is_enabled(mm))
>> +		__ptep_set_wrprotects(mm, addr, ptep, nr);
>> +	else if (nr == 1) {
> 
> Why do we need the special case here? Couldn't we just call
> contpte_set_wrprotects() with nr == 1?

My intention is for this to be a fast path for ptep_set_wrprotect(). I'm having
to work hard to prevent regressing the order-0 folios case.

> 
>> +		contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
>> +		__ptep_set_wrprotects(mm, addr, ptep, 1);
>> +		contpte_try_fold(mm, addr, ptep, __ptep_get(ptep));
>> +	} else
>> +		contpte_set_wrprotects(mm, addr, ptep, nr);
>> +}
>> +
>>  #define __HAVE_ARCH_PTEP_SET_WRPROTECT
>>  static inline void ptep_set_wrprotect(struct mm_struct *mm,
>>  				unsigned long addr, pte_t *ptep)
>>  {
>> -	contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
>> -	__ptep_set_wrprotect(mm, addr, ptep);
>> -	contpte_try_fold(mm, addr, ptep, __ptep_get(ptep));
>> +	ptep_set_wrprotects(mm, addr, ptep, 1);
>>  }
>>  
>>  #define __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS
>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c
>> index e079ec61d7d1..2a57df16bf58 100644
>> --- a/arch/arm64/mm/contpte.c
>> +++ b/arch/arm64/mm/contpte.c
>> @@ -303,6 +303,48 @@ int contpte_ptep_clear_flush_young(struct vm_area_struct *vma,
>>  }
>>  EXPORT_SYMBOL(contpte_ptep_clear_flush_young);
>>  
>> +void contpte_set_wrprotects(struct mm_struct *mm, unsigned long addr,
>> +					pte_t *ptep, unsigned int nr)
>> +{
>> +	unsigned long next;
>> +	unsigned long end = addr + (nr << PAGE_SHIFT);
>> +
>> +	do {
>> +		next = pte_cont_addr_end(addr, end);
>> +		nr = (next - addr) >> PAGE_SHIFT;
>> +
>> +		/*
>> +		 * If wrprotecting an entire contig range, we can avoid
>> +		 * unfolding. Just set wrprotect and wait for the later
>> +		 * mmu_gather flush to invalidate the tlb. Until the flush, the
>> +		 * page may or may not be wrprotected. After the flush, it is
>> +		 * guarranteed wrprotected. If its a partial range though, we
>> +		 * must unfold, because we can't have a case where CONT_PTE is
>> +		 * set but wrprotect applies to a subset of the PTEs; this would
>> +		 * cause it to continue to be unpredictable after the flush.
>> +		 */
>> +		if (nr != CONT_PTES)
>> +			contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
>> +
>> +		__ptep_set_wrprotects(mm, addr, ptep, nr);
>> +
>> +		addr = next;
>> +		ptep += nr;
>> +
>> +		/*
>> +		 * If applying to a partial contig range, the change could have
>> +		 * made the range foldable. Use the last pte in the range we
>> +		 * just set for comparison, since contpte_try_fold() only
>> +		 * triggers when acting on the last pte in the contig range.
>> +		 */
>> +		if (nr != CONT_PTES)
>> +			contpte_try_fold(mm, addr - PAGE_SIZE, ptep - 1,
>> +					 __ptep_get(ptep - 1));
>> +
>> +	} while (addr != end);
>> +}
>> +EXPORT_SYMBOL(contpte_set_wrprotects);
>> +
>>  int contpte_ptep_set_access_flags(struct vm_area_struct *vma,
>>  					unsigned long addr, pte_t *ptep,
>>  					pte_t entry, int dirty)
>