Measuring system call times, elapsed and cpu

[Stone, Joshua I]

Mike Mason wrote:
> I'm writing a script that attempts to measure how long it takes
> system calls to finish.  It uses gettimeofday_ns() to get the time at
> entry and exit, then calculates the delta.  Simple.  It occurred me,
> however, that a system call can start on one cpu and finish on
> another.  Now I'm wondering if I can rely on gettimeofday_ns() for
> this type of measurement.  If not, is there a better way to do this?

It should work, with the caveat of bug #3916.  The gettimeofday results
are system time, so they should be consistent across CPUs.

> I'd also like to break down the system call latency into cpu and wait
> times.  I thought of calculating the delta of utime+stime from
> task_struct to get cpu time, then subtracting that from the overall
> delta to get wait time.  Should this work or am I missing something?

This will include any time while you were context-switched away -- is
that your intention?


Josh