[PATCH 3/3] clocksource: exynos_mct: Optimize register reads with ldmia

[Doug Anderson]

Thomas,

On Wed, Jun 4, 2014 at 11:05 AM, Thomas Gleixner <tglx at linutronix.de> wrote:
> On Wed, 4 Jun 2014, Doug Anderson wrote:
>
>> As we saw in (clocksource: exynos_mct: cache mct upper count), the
>> time spent reading the MCT shows up fairly high in real-world
>> profiles.  That means that it's worth some optimization.
>>
>> We get a roughly 10% speedup in userspace gettimeofday() by using an
>> ldmia to read the two halfs of the MCT.  That seems like a worthwhile
>> thing to do.
>>
>> Before: 1173084 us for 1000000 gettimeofday in userspace
>> After:  1045674 us for 1000000 gettimeofday in userspace
>>
>> NOTE: we could actually do better than this if we really wanted to.
>> Technically we could register the clocksource as a 32-bit timer and
>> only use the "lower" half.  Doing so brings us down to 1014429 us for
>> 1000000 gettimeofday in userspace (and doesn't even require assembly
>> code).  That would be an alternative to this change.
>
> I was about to ask exactly that question: What's the advantage of the
> 64 bit dance there? AFAICT nothing.

Are you explicitly naking the 64-bit version of these patches?  If so
I'll send out the 32-bit version right away.  If nothing else we
should get the ftrace fix (patch 1 in this series) landed ASAP since
that fixes a regression.  I'd like to make the 32-bit decision first
though since fixing the regression is easier in the 32-bit version.

Roughly: with the 64-bit version there's no question that we're not
regressing anyone's functionality or performance and it's nearly as
fast as the 32-bit version.  The 32-bit version is simpler / faster
but has the potential to cause (unknown) problems.

-Doug