Re: [patch]x86: avoid unnecessary tlb flush

[Hugh Dickins]

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@xxxxxxxxx>
> 
> ---
>  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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