Loop-rate scheduled tasks always slip at 800Hz

[andyp1per]

I added a dummy callback to the runcam driver (doesn't matter where it is this was just convenient):

void AP_RunCam::dummy() {
}

I then included this in the vehicle schedule table at the loop rate:

SCHED_TASK_CLASS(AP_RunCam, &vehicle.runcam, dummy, LOOP_RATE, 50)

I then set SCHED_DEBUG = 2 and SCHED_LOOP_RATE = 800

The empty task is then reported to always slip:

Scheduler slip task[51-dummy] (2/1/50)
Scheduler slip task[51-dummy] (2/1/50)
Scheduler slip task[51-dummy] (2/1/50)
Scheduler slip task[51-dummy] (2/1/50)
Scheduler slip task[51-dummy] (2/1/50)
Scheduler slip task[51-dummy] (2/1/50)
Scheduler slip task[51-dummy] (2/1/50)
Scheduler slip task[51-dummy] (2/1/50)
Scheduler slip task[51-dummy] (2/1/50)
Scheduler slip task[51-dummy] (2/1/50)

My suspicion is that this is because 800Hz is close to the backend IMU rate of 1000Hz. 800Hz gives a tick length of 1250us and the IMU period is 1000us. Since we always wait for an IMU sample before scheduling the next tick if we overrun by 250us I think we will always slip loop rate tasks. Just a theory.

@tridge @peterbarker I could use some help figuring this out

Version
4.0.4-dev

Platform
[X] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Airframe type
All

Hardware type
Tested on an omnibusF4pro

[andyp1per]

Effectively fixed by #13758

[IamPete1]

fixed by #13741