[PATCH] ARM: implement optimized percpu variable access

[robherring2@gmail.com (Rob Herring)]

On 11/12/2012 10:51 AM, Will Deacon wrote:
> On Mon, Nov 12, 2012 at 02:41:22PM +0000, Will Deacon wrote:
>> On Mon, Nov 12, 2012 at 02:21:27PM +0000, Rob Herring wrote:
>>> On 11/12/2012 04:23 AM, Will Deacon wrote:
>>>> Hi Rob,
>>>>
>>>> On Sun, Nov 11, 2012 at 03:20:40AM +0000, Rob Herring wrote:
>>>>> From: Rob Herring <rob.herring@calxeda.com>
>>>>>
>>>>> Use the previously unused TPIDRPRW register to store percpu offsets.
>>>>> TPIDRPRW is only accessible in PL1, so it can only be used in the kernel.
>>>>>
>>>>> This saves 2 loads for each percpu variable access which should yield
>>>>> improved performance, but the improvement has not been quantified.
>>>>
>>>> The patch looks largely fine to me (one minor comment below), but we should
>>>> try and see what the performance difference is like on a few cores before
>>>> merging this. Have you tried something like hackbench to see if the
>>>> difference is measurable there? If not, I guess we'll need something more
>>>> targetted.
>>>
>>> Looks like it's about a 1.4% improvement on Cortex-A9 (highbank) with
>>> hackbench.
>>>
>>> Average of 30 runs of "hackbench -l 1000":
>>>
>>> Before: 6.2190666667
>>> After: 6.1347666667
>>>
>>> I'll add this data to the commit msg.
>>
>> Wow, that's really cool! I'll take it for a spin on 11MPCore to test the v6
>> angle...
> 
> Ok, similar numbers over here so it looks like this is definitely worth
> doing. However, I still object to the "cc", particularly after discussion
> with the tools guys here who agree that the behaviour you're seeing is
> indicative of a buggy compiler. It may even be part of a larger issue with
> GCC's definition of `reachability' for kernel entry points. For interest, I
> failed to reproduce with:
> 
>   gcc version 4.7.3 20121001 (prerelease) (crosstool-NG linaro-1.13.1-4.7-2012.10-20121022 - Linaro GCC 2012.10)
> (http://launchpad.net/linaro-toolchain-binaries/trunk/2012.10/+download/gcc-linaro-arm-linux-gnueabihf-4.7-2012.10-20121022_linux.tar.bz2)
> 
> which sounds fairly close to the tools that you are using. Please can you
> file a bug in launchpad? 

Strangely, I can't reproduce it either now...

Rob