Re: Cobalt scheduler preemption behavior

On 08.08.26 08:17, C Smith wrote:
> I am using Xenomai 3.1, Cobalt kernel 4.19.229 X8664  (I know it's old
> but this is a legacy system our company still develops).  I am
> wondering about expected Cobalt scheduler behavior on a single core. I
> have set isolcpus to reserve core 1, my first real time task sets
> affinity to core 1, the priority is 99.
> 
> ret = rt_task_create(&main_task, "my_rt_task", 0, 99, T_JOINABLE);
> CPU_ZERO(&cpus);
> CPU_SET(1, &cpus);
> rt_task_set_affinity(&main_task, &cpus)
> 
> Then I do the same with a second RT task, giving it the same priority
> (99) and same CPU core 1 affinity.
> 
> First Question: If the first RT task is executing, and the second RT
> task wakes up from a sleep, can I expect the scheduler to preempt the
> first RT task to run the second RT task on that same CPU core?

Same prio and SCHED_FIFO (used when prio > 0) means that the first task
runs until completion (exit, self-suspend, blocked on wait-condition)
before the second in that FIFO queue for the same prio level is run.

> 
> I put DIO pulses around both tasks and the oscilloscope says 'yes',
> the second task preempts the first. I'm a bit confused about that, as
> I expected FIFO non-preemptable behavior from the Cobalt kernel.  ie:
> I thought the first task would finish, then the second task would run.
> 
> Second question: Is there a way to configure the Cobalt kernel, or my
> two RT tasks, such that they can't preempt one another, but when there
> is contention the first to run is allowed to finish before the second
> runs?
> 

To achieve preemption, you need different prio levels or some
time-slicing configuration (rt_task_slice).

Jan

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