linux-kernel - Re: [PATCH V2] arm64: Don't flush tlb while clearing the accessed bit

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Date:   Mon, 29 Oct 2018 08:57:50 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Ashish Mhetre <amhetre@...dia.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-tegra@...r.kernel.org,
        avanbrunt@...dia.com, linux-kernel@...r.kernel.org,
        vdumpa@...dia.com
Subject: Re: [PATCH V2] arm64: Don't flush tlb while clearing the accessed bit

On Mon, Oct 29, 2018 at 11:59:59AM +0530, Ashish Mhetre wrote:
> From: Alex Van Brunt <avanbrunt@...dia.com>
> 
> Accessed bit is used to age a page and in generic implementation there is
> flush_tlb while clearing the accessed bit.
> Flushing a TLB is overhead on ARM64 as access flag faults don't get
> translation table entries cached into TLB's. Flushing TLB is not necessary
> for this. Clearing the accessed bit without flushing TLB doesn't cause data
> corruption on ARM64. [It may cause incorrect page aging but chances of that
> should be relatively low.]
> In our case with this patch, speed of reading from fast NVMe/SSD through
> PCIe got improved by 10% ~ 15% and writing got improved by 20% ~ 40%.
> So for performance optimisation don't flush TLB when clearing the accessed
> bit on ARM64.
> x86 made the same optimization even though their TLB invalidate is much
> faster as it doesn't broadcast to other CPUs.

Please specifically refer to commit:

  b13b1d2d8692b437 ("x86/mm: In the PTE swapout page reclaim case clear the accessed bit instead of flushing the TLB")

... so that it's easy for people to track down the relevant x86 change.

> 
> Signed-off-by: Alex Van Brunt <avanbrunt@...dia.com>
> Signed-off-by: Ashish Mhetre <amhetre@...dia.com>
> ---
> v2: Added comments about why flushing is not needed while clearing accessed bit
> 
>  arch/arm64/include/asm/pgtable.h | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 2ab2031..33e1940 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -652,6 +652,22 @@ static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
>  	return __ptep_test_and_clear_young(ptep);
>  }
>  
> +#define __HAVE_ARCH_PTEP_CLEAR_YOUNG_FLUSH
> +static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
> +					 unsigned long address, pte_t *ptep)
> +{
> +	/*
> +	 * Flushing a TLB is overhead on ARM64 as access flag faults don't get
> +	 * translation table entries cached into TLB's. Flushing TLB is not
> +	 * necessary for this. Clearing the accessed bit without flushing TLB
> +	 * doesn't cause data corruption on ARM64.[ It may cause imcorrect page
> +	 * aging but chances of this should be comparatively low. ]
> +	 * So as a performance optimization don't flush the TLB when clearing
> +	 * the accessed bit.
> +	 */

Can we just copy the x86 comment from commit b13b1d2d8692b437?

Thanks,
Mark.

> +	return ptep_test_and_clear_young(vma, address, ptep);
> +}
> +
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  #define __HAVE_ARCH_PMDP_TEST_AND_CLEAR_YOUNG
>  static inline int pmdp_test_and_clear_young(struct vm_area_struct *vma,
> -- 
> 2.7.4
>