linux-kernel - Re: [PATCH 2/7] iommu/iova: cut down judgement times

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Date:   Fri, 31 Mar 2017 11:55:38 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Sudeep Dutt <sudeep.dutt@...el.com>,
        Ashutosh Dixit <ashutosh.dixit@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
CC:     Zefan Li <lizefan@...wei.com>, Xinwei Hu <huxinwei@...wei.com>,
        "Tianhong Ding" <dingtianhong@...wei.com>,
        Hanjun Guo <guohanjun@...wei.com>
Subject: Re: [PATCH 2/7] iommu/iova: cut down judgement times



On 2017/3/23 20:11, Robin Murphy wrote:
> On 22/03/17 06:27, Zhen Lei wrote:
>> Below judgement can only be satisfied at the last time, which produced 2N
>> judgements(suppose N times failed, 0 or 1 time successed) in vain.
>>
>> if ((pfn >= iova->pfn_lo) && (pfn <= iova->pfn_hi)) {
>> 	return iova;
>> }
> 
> For me, GCC (6.2.1 AArch64) seems to do a pretty good job of this
> function already, so this change only saves two instructions in total
> (pfn is compared against pfn_lo only once instead of twice), which I
> wouldn't expect to see a noticeable performance effect from.
OK, thanks for your careful analysis.

Although only two instructions saved in each loop iteration, but it's also an improvment and no harm.

> 
> Given the improvement in readability, though, I don't even care about
> any codegen differences :)
> 
> Reviewed-by: Robin Murphy <robin.murphy@....com>
> 
>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>> ---
>>  drivers/iommu/iova.c | 9 +++------
>>  1 file changed, 3 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
>> index 8ba8b496..1c49969 100644
>> --- a/drivers/iommu/iova.c
>> +++ b/drivers/iommu/iova.c
>> @@ -312,15 +312,12 @@ private_find_iova(struct iova_domain *iovad, unsigned long pfn)
>>  	while (node) {
>>  		struct iova *iova = rb_entry(node, struct iova, node);
>>  
>> -		/* If pfn falls within iova's range, return iova */
>> -		if ((pfn >= iova->pfn_lo) && (pfn <= iova->pfn_hi)) {
>> -			return iova;
>> -		}
>> -
>>  		if (pfn < iova->pfn_lo)
>>  			node = node->rb_left;
>> -		else if (pfn > iova->pfn_lo)
>> +		else if (pfn > iova->pfn_hi)
>>  			node = node->rb_right;
>> +		else
>> +			return iova;	/* pfn falls within iova's range */
>>  	}
>>  
>>  	return NULL;
>>
> 
> 
> .
> 

-- 
Thanks!
BestRegards