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I want to measure time intervals in my kernel. I know nanoseconds are not really precise but I need more precision than milliseconds.

There are many many functions to get a time. Jiffies, time of day, time without sleeping, time with sleeping... There is ktime_get, clock_gettime, sys_clock_gettime and others.
I looked into the functions and feel like some enormous machinery is started in them. I don't need conversion to time of day and I also want to avoid converting values back and forth from and to several time-keeping structs.

For a start without respecting performance I thought I am fine with

struct compat_timespec cts;
compat_sys_clock_gettime(CLOCK_MONOTONIC, &cts);

It worked until I wanted to use it in a file open function of a virtual file. Now compat_sys_clock_gettime returns an error code. What can cause an error reading a time?

So my question is embarrassingly basic.

How can I performantly just get the nanoseconds since booting the PC...

  • without subtracting sleep times or any other special conditions,
  • without a possibility of failure?

My simple imagination is there should be a function that reads a ticker value from somewhere and multiplies / divides this value to get ns out of it. I'd also appreciate to learn why it's not so easy.

  • You could find a hardware source (such as the powerpc time base register - see this) - but sounds to me like using jiffies would be good enough - you can get a timespec from jiffies using jiffies_to_timespec(). Have a read of this. – Murray Jensen Jun 25 at 2:33
  • @MurrayJensen Thanks for this hint but jiffies is too coarse. This is on a small device, I don't know the exact value right now but the jiffies rate is far below 1000. This resolution is larger than a millisecond but must be more exact. The function in my example would be great if it didn't return an error in some cases. Strange to see reading a simple ticker is such a rocket science. – ThreadGuy Jun 25 at 4:58
  • You are making a simple thing complicated by calling a user process system call in your kernel code (generally speaking, things that start with sys_ are often system calls - haven't seen compat_sys_ before, but probably the same ... it is, see this). If jiffies are too coarse for you, you will have to read a hardware time base register (which makes your code platform dependent). Maybe there is a platform independent way to do this (I would like to know). – Murray Jensen Jun 25 at 9:49
  • @MurrayJensen Oh a good point, I didn't thinkk about sys_. I wrote an answer how I do this now.. – ThreadGuy Jun 25 at 13:18
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As of lately I am using ktime_get.

I have no idea if there is something better or if this function is so common it's a good idea to use it. But this function seems to return a small resolution and is at least not incredibly slow.
Also it has no return value which implies it shouldn't fail but reliably return a time.

  • Thanks for the pointer - this looks to me like the right thing to use (read this) – Murray Jensen Jun 25 at 13:41

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