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SubjectRe: [patch]x86: avoid unnecessary tlb flush
On Fri, 13 Aug 2010, Shaohua Li wrote:
>
> In x86, access and dirty bits are set automatically by CPU when CPU accesses
> memory. When we go into the code path of below flush_tlb_nonprotect_page(),
> we already set dirty bit for pte and don't need flush tlb. This might mean
> tlb entry in some CPUs hasn't dirty bit set, but this doesn't matter. When
> the CPUs do page write, they will automatically check the bit and no software
> involved.
>
> On the other hand, flush tlb in below position is harmful. Test creates CPU
> number of threads, each thread writes to a same but random address in same vma
> range and we measure the total time. Under a 4 socket system, original time is
> 1.96s, while with the patch, the time is 0.8s. Under a 2 socket system, there is
> 20% time cut too. perf shows a lot of time are taking to send ipi/handle ipi for
> tlb flush.
>
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
>
> ---
> arch/x86/include/asm/pgtable.h | 2 ++
> include/asm-generic/pgtable.h | 4 ++++
> mm/memory.c | 2 +-
> 3 files changed, 7 insertions(+), 1 deletion(-)
>
> Index: linux/arch/x86/include/asm/pgtable.h
> ===================================================================
> --- linux.orig/arch/x86/include/asm/pgtable.h 2010-08-13 08:23:13.000000000 +0800
> +++ linux/arch/x86/include/asm/pgtable.h 2010-08-13 08:24:53.000000000 +0800
> @@ -603,6 +603,8 @@ static inline void ptep_set_wrprotect(st
> pte_update(mm, addr, ptep);
> }
>
> +#define flush_tlb_nonprotect_page(vma, address)
> +
> /*
> * clone_pgd_range(pgd_t *dst, pgd_t *src, int count);
> *
> Index: linux/include/asm-generic/pgtable.h
> ===================================================================
> --- linux.orig/include/asm-generic/pgtable.h 2010-08-13 08:23:13.000000000 +0800
> +++ linux/include/asm-generic/pgtable.h 2010-08-13 08:24:53.000000000 +0800
> @@ -129,6 +129,10 @@ static inline void ptep_set_wrprotect(st
> #define move_pte(pte, prot, old_addr, new_addr) (pte)
> #endif
>
> +#ifndef flush_tlb_nonprotect_page
> +#define flush_tlb_nonprotect_page(vma, address) flush_tlb_page(vma, address)
> +#endif
> +
> #ifndef pgprot_noncached
> #define pgprot_noncached(prot) (prot)
> #endif
> Index: linux/mm/memory.c
> ===================================================================
> --- linux.orig/mm/memory.c 2010-08-13 08:23:13.000000000 +0800
> +++ linux/mm/memory.c 2010-08-13 08:24:53.000000000 +0800
> @@ -3116,7 +3116,7 @@ static inline int handle_pte_fault(struc
> * with threads.
> */
> if (flags & FAULT_FLAG_WRITE)
> - flush_tlb_page(vma, address);
> + flush_tlb_nonprotect_page(vma, address);
> }
> unlock:
> pte_unmap_unlock(pte, ptl);

Just added Andrea to the Cc list: he did that TLB flush in 1a44e149,
I'd feel more comfortable noop-ing it on x86 if you've convinced him.

Hugh


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