[lttng-dev] Using lttng-ust with xenomai

[Jan Kiszka]

On 22.11.19 18:01, Mathieu Desnoyers wrote:
>>>>
>>>> ## membarrier syscall
>>>>
>>>> I haven't got an explanation yet, but I believe this syscall does
>>>> nothing to xenomai threads (each has a shadow linux thread, that is
>>>> *idle* when the xenomai thread is active).
>>>
>>> That's indeed a good point. I suspect membarrier may not send any IPI
>>> to Xenomai threads (that would have to be confirmed). I suspect the
>>> latency introduced by this IPI would be unwanted.
>>
>> Is an "IPI" a POSIX signal here? Or are real IPI that delivers an
>> interrupt to Linux on another CPU? The latter would still be possible,
>> but it would be delayed until all Xenomai threads on that core eventual
>> took a break (which should happen a couple of times per second under
>> normal conditions - 100% RT load is an illegal application state).
> 
> I'm talking about a real in-kernel IPI (as in inter-processor interrupt).
> However, the way sys_membarrier detects which CPUs should receive that IPI
> is by iterating on all cpu runqueues, and figure out which CPU is currently
> running a thread which uses the same mm as the sys_membarrier caller
> (for the PRIVATE membarrier commands).
> 
> So I suspect that the Xenomai thread is really not within the Linux scheduler
> runqueue when it runs.

True. Xenomai first suspends the RT thread's Linux shadow and then kicks 
the Xenomai scheduler to interrupt Linux (and schedule in the RT 
thread). So, from a remote Linux perspective, something else will be 
running at this point.

Jan

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