Preempt-rt

preempt-rt are a series of patches to improve Linux kernel realtime performances. This page explain how to use it.

Installation

• Depending on your board, you may have a Buildroot defconfig to apply preempt-rt patches automagically. For example for OPOS6UL:

$ make opos6ul-preempt-rt_defconfig

• If no config is available, you will have to do the job yourself, by finding a kernel version compatible with preempt-rt patches from https://www.kernel.org/pub/linux/kernel/projects/rt/

Usage

• You can change realtime priority of tasks with chrt. For example to set process X to priority 20 in FIFO scheduling queue, you have to do:

# chrt -p -f 20 X

pid X's current scheduling policy: SCHED_OTHER
pid X's current scheduling priority: 0
pid X's new scheduling policy: SCHED_FIFO
pid X's new scheduling priority: 20

• The more priority you set, the more your process is prioritary.
• Priority queues are OTHER (no realtime), FIFO (First In First Out), RR (Round Robin time share)
• You can validate realtime performances of your system with cyclictest. For example to launch 10 processes triggering on timer set to 10ms with different descending priorities, you can do:

# cyclictest -t 10 -p 20 -n -i 10000 --priospread

/dev/cpu_dma_latency set to 0us
policy: fifo: loadavg: 0.45 0.23 0.10 1/86 230          

T: 0 (  219) P:99 I:10000 C:  34012 Min:     67 Act:  166 Avg:  174 Max:     536
T: 1 (  220) P:98 I:10500 C:  32393 Min:     76 Act:  228 Avg:  202 Max:     719
T: 2 (  221) P:97 I:11000 C:  30920 Min:     66 Act:  170 Avg:  197 Max:    1154
T: 3 (  222) P:96 I:11500 C:  29576 Min:     65 Act:  217 Avg:  194 Max:    1176
T: 4 (  223) P:95 I:12000 C:  28343 Min:     72 Act:  167 Avg:  194 Max:    1912
...

• times are in microseconds

Links

• https://wiki.linuxfoundation.org/realtime/documentation/howto/applications/preemptrt_setup
• https://wiki.linuxfoundation.org/realtime/documentation/howto/applications/application_base