added funclatency, a tool for summarizing kernel function latencies

[ryao]

I really wanted histograms on function execution times, so I modified iolatency to get it.

Signed-off-by: Richard Yao ryao@gentoo.org

[brendangregg]

Sorry for the delay. I think this tool is simply too dangerous in terms of overhead to include, as it's passing every entry and return event to user space for post-processing. Users should be using BPF, which can do the summaries in-kernel, or at the very least, the ftrace capabilities for average latency.

I understand that people on older kernels may have no choice. I'd consider including this tool if by default it would not run, and printed a message saying something like:

This tool is a high-overhead hack for older kernels, that works by sending all function entry and return events to user space for post-processing. Please use tools based on BPF and Linux 4.x series (or newer) kernels to measure this efficiently. If you are desperate, and are willing to pay what may be severe overhead, and will test in a lab environment before use, then you can continue with -F to force. It's not recommend. You have been warned.

And implement a -F.

[brendangregg]

Closing; this is a job for BPF, or, post-processing funcslower(8).