Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL

From	"Jon Medhurst (Tixy)" <tixy@linaro.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL
Date	2016-01-20 17:10 +0100
Message-ID	<qT3uz-vq-51@gated-at.bofh.it> (permalink)
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On Wed, 2016-01-20 at 14:46 +0000, Robin Murphy wrote:
> > Of course, the 0.004014s maybe not accurate enough, it is just an
> > approximate number.
> 
> A mean and standard deviation of at least, say, 5 runs each with and 
> without the patch would be considerably more meaningful (even if
> still 
> far from statistically significant).

It wouldn't surprise me if replacing the proposed change with an 'asm
volatile("nop")' or two also give a boot time delta of several
milliseconds (due to change in cache line alignment of functions). I
don't believe you can reliably measure such minor changes.

It doesn't mean that the proposed change isn't a good addition though,
it obviously results in less code getting executed for the cost of one
or two instructions for a compare and branch. 

-- 
Tixy

Thread

Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL Joerg Roedel <joro@8bytes.org> - 2016-01-20 13:10 +0100
  Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL Huang Shijie <shijie.huang@arm.com> - 2016-01-20 14:40 +0100
    Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL Robin Murphy <robin.murphy@arm.com> - 2016-01-20 15:50 +0100
      Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is  NULL "Jon Medhurst (Tixy)" <tixy@linaro.org> - 2016-01-20 17:10 +0100
      Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is NULL Huang Shijie <shijie.huang@arm.com> - 2016-01-21 03:10 +0100

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