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

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

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?

If so, we need to convert "expire time" into jiffies time, but
I searched ways for this type of converting but found nothing.

I'm afraid it's hard to do as below example:
  insmod-3821  [001]  3192.239335: hrtimer_start: timer=d08a1480  expires=1245162841000000000 ns
  <idle>-0     [001]  3201.506127: hrtimer_expire: timer=d08a1480

We can't convert 1245162841000000000ns to jiffies if we don't know
the xtime/wall_to_monotonic at hrimter_start.
And the expire time that got by _start() maybe wrong because user
can change time of day between hrtimer_start() and hrtimer_expire().

What your opinion?

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