Editing Rate-monotonic scheduling (section)

=== Mitigating mis-prioritized ISRs ===

One method for mitigating a mis-prioritized ISR is to adjust the analysis by reducing the ISR's period to be equal to that of the shortest period, if possible.  Imposing this shorter period results in prioritization that conforms to RMS, but also results in a higher utilization factor for the ISR and therefore for the total utilization factor, which may still be below the allowable bound and therefore schedulability can be proven.  As an example, consider a hardware ISR that has a computation time, <math>{C_{isr}}</math> of 500 microseconds and a period, <math>{T_{isr}}</math>, of 4 milliseconds.  If the shortest scheduler-controlled task has a period, <math>{T_1}</math> of 1 millisecond, then the ISR would have a higher priority, but a lower rate, which violates RMS.  For the purposes of proving schedulability, set <math>{T_{isr}}={T_1}</math> and recalculate the utilization factor for the ISR (which also raises the total utilization factor).  In this case, <math>{U_{isr}}{=}{C_{isr}}/{T_{isr}}</math> will change from <math>{0.5 ms}/{4 ms}{=}0.125</math> to <math>{0.5 ms}/{1 ms}{=}0.5</math>.  This utilization factor would be used when adding up the total utilization factor for the task set and comparing to the upper bound to prove schedulability.  It should be emphasized that adjusting the period of the ISR is for analysis only and that the true period of the ISR remains unchanged.

Another method for mitigating a mis-prioritized ISR is to use the ISR to only set a new semaphore/mutex while moving the time-intensive processing to a new process that has been appropriately prioritized using RMS and will block on the new semaphore/mutex.  When determining schedulability, a margin of CPU utilization due to ISR activity should be subtracted from the least upper bound.  ISRs with negligible utilization may be ignored.