Add --syscalls option for tracing syscalls (linux only)

[manuels]

This commit adds a new option (--syscalls/-S) to trace what time your python programm spends in kernel space.
It is only supported on linux where current syscall can be determined using /proc/{pid}/task/{tid}/syscall.
--syscalls implies --idle

[manuels]

Example output of top:

  %Own   %Total  OwnTime  TotalTime  Function (filename:line)                                   
 99.00%  99.00%   67.60s    67.60s   select syscall (<kernel>)
  1.00%   1.00%   0.200s    0.200s   write syscall (<kernel>)
  0.00%   1.00%   0.130s    0.390s   x (x.py:5)
  0.00%   0.00%   0.060s    0.060s   brk syscall (<kernel>)
  0.00%   0.00%   0.010s    0.010s   x (x.py:3)
  0.00%  99.00%   0.000s    67.60s   x (x.py:4)
  0.00% 100.00%   0.000s    68.00s   <module> (x.py:7)

[Jongy]

Nice!

I had something similar in mind a while back - to add /proc/pid/stack entries as traces (so you have kernel stacks in addition to the Python stacks). It's simpler (because the kernel does symbolication for you) but requires root IIRC (unlike /proc/pid/syscall).

Also, I'm not sure it's useful unless you are in --nonblocking mode - otherwise, the kernel stack would be "stopped in ptrace".

[manuels]

Yes, my first idea was to add /proc/pid/stack, too, but then I thought that a programmer that only sees the python stacktrace (and not the c stacktrace of cpython) is probably barely interested in the internals of the kernel ;)

[benfred]

Thanks for the PR!

[manuels]

Hey, @benfred, let me know if there is anything in this pull request that I should change if you are not happy with the current state. :D

[manuels]

Hey, @benfred is there anything else I can do to have this merged?

[manuels]

Hey, @benfred, maybe you missed my last message. Is there anything else I can do to have this merged?