[PATCH] arm: dts: exynos5: Remove multi core timer

[Doug Anderson]

Tomasz,

On Thu, May 15, 2014 at 4:25 PM, Tomasz Figa <tomasz.figa at gmail.com> wrote:
> On 16.05.2014 01:18, David Riley wrote:
>> On Thu, May 15, 2014 at 4:03 PM, Chirantan Ekbote
>> <chirantan at chromium.org> wrote:
>>> Hi Tomasz,
>>>
>>> On Thu, May 15, 2014 at 3:44 PM, Doug Anderson <dianders at chromium.org> wrote:
>>>> Tomasz,
>>>>
>>>> On Thu, May 15, 2014 at 3:13 PM, Tomasz Figa <tomasz.figa at gmail.com> wrote:
>>>>>> NOTE: if for some reason we need to keep the MCT around, we're
>>>>>> definitely going to need to account for the fact that tweaking it
>>>>>> affects the arch timer.  ...and having the arch timer is really nice
>>>>>> since:
>>>>>
>>>>> [Let me reorder the points below to make it easier to comment:]
>>>>>
>>>>>> * it's faster to access.
>>>>>> * it is accessible from userspace for really fast access.
>>>>>
>>>>> Do you have some data on whether it is a significant difference,
>>>>> especially considering real use cases?
>>>>
>>>> I know that Chrome makes _a lot_ of calls to gettimeofday() for
>>>> profiling purposes, enough that it showed up on benchmarks.  In fact,
>>>> we made a change to the MCT to make accesses faster and there's a
>>>> small mention of the benchmarking that was done at:
>>>>
>>>> https://chromium-review.googlesource.com/#/c/32190/
>>>>
>>>> ...that change probably should be sent upstream, actually.
>>>>
>>>> I'll let Chirantan comment on how much faster arch timers were.
>>>> ...and I think David Riley (also CCed now) may be able to comment on
>>>> the benefits of userspace timers.
>>>>
>>>
>>> When I profiled gettimeofday() calls, they were about 50 - 60% faster
>>> with the arch timers compared to the mct.
>>
>> When I profiled gettimeofday(), the standard systems call version took
>> about 2.5x longer than through a vDSO interface.
>
> Sounds like we should invent a new kind of jokes, starting with "When I
> profiled gettimeofday()". ;)
>
> Just kidding.
>
> The raw improvement looks quite good, but what I'm more concerned about
> is whether this has any significant effect on real use cases. As Doug
> mentioned, Chrome makes a lot of calls to gettimeofday(), but he also
> mentioned that this is for profiling purposes. Is performance of
> gettimeofday() really that crucial in this case? Are there any other use
> cases when this improvement is significant?

I guess I should restate.  Chrome is always profiling to some extent.
I think that the Javascript engine, for instance, uses gettimeofday()
to figure out how much time it's spending in various places.

Sonny just sent me some recent benchmarks using perf.  On this
particular benchmark it shows 1.85% of the time was spent in
exynos_frc_read.  If we can half that then we'll get a ~1% speedup in
the system, which is nothing to sneeze at.  If getting rid of the
system call overead makes this several times faster again, then we
roughly eliminate the overhead totally.


> Anyway, I'm by no means opposed to switching to arch timers. They
> provide a well designed, generic interface and drivers shared by
> multiple platforms, which means more code sharing and possibly more eyes
> looking at the code, which is always good. However if they don't support
> low power states correctly, we can't just remove MCT.

I think low power states aren't in mainline (right?).

One solution that might work could be to leave the device tree entry
alone but change the MCT init code to simply act as a no-op if it sees
an arch timer is in the device tree and enabled.  Then when/if someone
got the low power states enabled we could just change source code
rather than dts files.

-Doug