Re: [PATCH 1/3] clocksource: exynos_mct: Fix ftrace

[Daniel Lezcano <daniel.lezcano@linaro.org>]

On 06/16/2014 06:40 AM, Doug Anderson wrote:
> Daniel,
>
> On Sun, Jun 15, 2014 at 2:18 PM, Daniel Lezcano
> <daniel.lezcano@linaro.org> wrote:
>> On 06/04/2014 07:30 PM, Doug Anderson wrote:
>>>
>>> In (93bfb76 clocksource: exynos_mct: register sched_clock callback) we
>>> supported using the MCT as a scheduler clock.  We properly marked
>>> exynos4_read_sched_clock() as notrace.  However, we then went and
>>> called another function that _wasn't_ notrace.  That means if you do:
>>>
>>>     cd /sys/kernel/debug/tracing/
>>>     echo function_graph > current_tracer
>>>
>>> You'll get a crash.
>>>
>>> Fix this (but still let other readers of the MCT be trace-enabled) by
>>> adding an extra function.  It's important to keep other users of MCT
>>> traceable because the MCT is actually quite slow.
>>
>>
>>
>> Hi Doug,
>>
>> could you elaborate ? I don't get the 'because the MCT ... slow'
>
> Sorry, I was trying to avoid duplication in the series and it's more
> obvious when you look at parts 2 and 3 of the series.  ;)
>
> Doing the math (please correct any miscalculations) using the numbers
> from the other patches: You can see that the existing code takes
> 1323852 us for 1000000 gettimeofday in userspace.  The fastest
> implementation (just shaving to a 32-bit timer) gets us as fast as
> ~1000000 us for 1000000 gettimeofday in userspace.
>
>  From profiling, I believe that gettimeofday from userspace is about
> 50% overhead (system call, multiplication, copies, etc) and about 50%
> MCT read.  That means that the fastest you can possibly do an MCT read
> is in .5us or 500ns.
>
> I believe an A15 has something like 1 or 2 cycles per instruction.  If
> it were 2 cycles per instruction, it can execute a normal instruction
> on a 2GHz machine in .5ns.  That means we can execute 1000 normal
> instructions in the time it takes to do a since MCT access.
>
> ...so I guess that's what I'd call slow.  ;)  What do you think?  I
> know that the MCT read shows up in whole system profiles of
> gettimeofday.

Hi Dough,

thanks for the explanation. I still don't get why it is important to 
keep others users of mct traceable because it is quite slow ? May be it 
is what you explained here, but I miss the connection between 'the other 
users' <-> 'traceable' <-> 'because slow'.

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