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

[Alistair Popple <apopple@nvidia.com>]

Ryan Roberts <ryan.roberts@arm.com> 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@arm.com>
> ---
>  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?

> +		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)