Try using SCHED_FIFO on any kernel

[fmauch]

A while back, we did some deeper investigation of cycle-time effects of different kernel / scheduling setups (https://github.com/UniversalRobots/Universal_Robots_ROS_Driver/blob/60f08359f6484d1671e12bab6e01fc49eb0fadc8/ur_robot_driver/doc/real_time_benchmarking.md).

Based on this, using SCHED_FIFO is beneficial on any system, not only real-time kernels. Thus, I suggest alwats trying FIFO scheduling in the CM.

I've swapped the user setup before the kernel setup in the userdocs for two reasons:

• Since the CM now will put a warning about FIFO scheduling in every run, the method of avoiding this warning should be put first
• Users might be repelled from installing another kernel, especially installing an RT-kernel. (At least that's our experience from issue reports). This should not necessarily stop them from setting up their ROS user to use SCHED_FIFO.

[codecov]

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@@           Coverage Diff           @@
##           master    #1142   +/-   ##
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  Coverage   31.62%   31.63%           
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  Files          94       94           
  Lines       10838    10836    -2     
  Branches     7419     7419           
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  Hits         3428     3428           
+ Misses        804      802    -2     
  Partials     6606     6606           

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Files	Coverage Δ
controller_manager/src/ros2_control_node.cpp	0.00% <0.00%> (ø)

[olivier-stasse]

LGTM

[destogl]

🎉❣️