On 15/12/2023 04:32, Alistair Popple wrote: > > Ryan Roberts <ryan.roberts@xxxxxxx> writes: > >> 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. > > This ends up calling three functions anyway so I'm curious - does > removing the one function call really make that much of difference? Yes; big time. All the functions in the fast path are inlined. The version regresses a fork() microbenchmark that David gave me by ~30%. I've had to work quite hard to reduce that to 2%, even from this starting point. There is so little in the inner loop that even the __ptep_get(ptep) (which is a READ_ONCE()) makes a measurable difference. Anyway, I'll be posting v4 with these optimizations and all the supporting benchmark data on Monday. > > Either way I think a comment justifying the special case (ie. that this > is simply a fast path for nr == 1) would be good. I've added a comment here in v4. > > Thanks. > >>> >>>> + 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) >>> >
