linux-kernel - Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev

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Date:	Wed, 20 Jan 2016 16:00:01 +0000
From:	"Jon Medhurst (Tixy)" <tixy@...aro.org>
To:	Robin Murphy <robin.murphy@....com>
Cc:	Huang Shijie <shijie.huang@....com>,
	Joerg Roedel <joro@...tes.org>, will.deacon@....com,
	iommu@...ts.linux-foundation.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu: add a shortcut when the @dev_node is
 NULL

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