I want to get Linux to respond to events within 2.5us, and the RT kernel seems to be able to do this. I know that the RT kernel has preemption, and so syscalls can be interrupted. However, when I run a cyclictest, I’m finding that the average response time is 53us, while the minimum is 2us. My CPU has a minimum frequency of 800MHz (1.25ns). Only one task needs to have priority over everything else, and once it is running, it will block other tasks from taking control until it is completed for a short period of time (maybe 100us). What can I do to decrease the latency? Is the 2us minimum my context-switching cost, or can that be even lower? Can I also guarantee that a process will be executed in a 10ms time window with Linux RT? Thanks.
sudo cyclictest --smp
# /dev/cpu_dma_latency set to 0us
policy: other/other: loadavg: 1.33 1.00 0.73 1/812 6256
T: 0 ( 6185) P: 0 I:1000 C: 151177 Min: 3 Act: 53 Avg: 56 Max: 9369
T: 1 ( 6186) P: 0 I:1500 C: 100878 Min: 2 Act: 53 Avg: 56 Max: 9277
T: 2 ( 6187) P: 0 I:2000 C: 75667 Min: 3 Act: 54 Avg: 61 Max: 8991
T: 3 ( 6188) P: 0 I:2500 C: 60539 Min: 4 Act: 53 Avg: 57 Max: 8689
A clue I’ve found is in Oliveira et al. (2020) have a tool with accompanying code that seems to model the scheduling latency of the RT kernel:
L_IF ≤ max(D_ST, D_POID) + D_PAIE + D_PSD
- L means the scheduling latency;
- IF Interference-free;
- D means the delay of:
** ST: the sched tail delay which is the delay from the IRQs being disabled to cause the context switch, and the return from the scheduler.
** POID: the longest preemption or IRQ disabled to postpone the scheduler;
** PAIE: the longest time in which the preemption and IRQs are transiently enabled in the return of the preemption or IRQ enable, that will cause the scheduler execution to preempt the current thread.
** PSD: the longest time in which the preemption is disabled to execute __schedule() function.
But, there are no numbers in the paper, and I am not sure what I can change in the kernel to alter these values. Anyone have any RT kernel experience?