linux-kernel - Re: [PATCH V3] arm64: Don't flush tlb while clearing the accessed bit

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Message-ID: <b014451f-d81e-1d02-4b04-f719bca6621d@nvidia.com>
Date:   Mon, 29 Oct 2018 09:57:04 +0000
From:   Jon Hunter <jonathanh@...dia.com>
To:     Ashish Mhetre <amhetre@...dia.com>, <mark.rutland@....com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-tegra@...r.kernel.org>, <avanbrunt@...dia.com>
CC:     <linux-kernel@...r.kernel.org>, <vdumpa@...dia.com>,
        <Snikam@...dia.com>
Subject: Re: [PATCH V3] arm64: Don't flush tlb while clearing the accessed bit


On 29/10/2018 09:25, 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.
> 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 refer to:
> 'commit b13b1d2d8692 ("x86/mm: In the PTE swapout page reclaim case clear
> the accessed bit instead of flushing the TLB")'
> 
> Signed-off-by: Alex Van Brunt <avanbrunt@...dia.com>
> Signed-off-by: Ashish Mhetre <amhetre@...dia.com>
> ---

Please make sure you state here below the above line what has been
changed between each version of the patch.

Thanks
Jon

-- 
nvpublic