Linux Timing across Kernel & User Space

Question

I'm writing a kernel module for a special camera, working through V4L2 to handle transfer of frames to userspace code.. Then I do lots of userspace stuff in the app..

Timing is very critical here, so I've been doing lots of performance profiling and plain old std::chrono::steady_clock stuff to track timing, but I've reached the point where I need to also collect timing data from the Kernel side of things so that I can analyze the entire path from hardware interrupt through V4L DQBuf to userspace...

Can anyone recommend a good way to get high-resolution timing data, that would be consistent with application userspace data, that I could use for such comparisons? Right now I'm measuring activity in microseconds..

Ubuntu 12.04 LTS

Answer 1

At the lowest level, there are the rdtsc and rdtscp instructions if you're on an x86/x86-64 processor. That should provide the lowest overhead, highest possible resolution across the kernel/userspace boundary.

However, there are things you need to worry about. You need to make sure you're executing across the same core/cpu, the process isn't being context switched, and the frequency isn't changing across invocations. If the cpu supports an invariant tsc, (constant_tsc in /proc/cpuinfo) it's a little more reliable across cpus/cores and frequencies.

This should provide roughly nanosecond accuracy.