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) >
