Re: [PATCH v3 3/4] ftrace: add tracepoint for hrtimer

[Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>]

Peter Zijlstra wrote:
> On Thu, 2009-07-23 at 18:01 +0800, Xiao Guangrong wrote:
>> Peter Zijlstra wrote:
>>
>>> OK, so what you want to measure is the time of the actual callback
>>> happening (hrtimer_entry) vs that where you would have expected it to
>>> happen (hrtimer_start + delay), right?
>>>
>> Yes
>>
>>> So what's wrong with printing the expected expiration time in the
>>> hrtimer_start tracepoint in the cheap clock units?
>>>
>> Is "cheap clock units" means jiffies time?
> 
> Nah, something like cpu_clock() which is monotonic per-cpu and should
> have high resolution where available.
> 

Hi Peter,

Thanks for your valuable suggestion.

I have do some test for it, and it very suitable for CLOCK_MONOTONIC clock,
but if the hrtimer's clock is CLOCK_REALTIME, however xtime is needed to get
the real time.

In addition, it make TP_fast_assign() become a slow path, we should avoid it
if possible.

Thanks,
Xiao

>