[lttng-dev] Using lttng-ust with xenomai

Norbert Lange nolange79 at gmail.com
Fri Nov 22 12:55:38 EST 2019

> LTTng-UST prepares the ring buffers from lttng-ust's "listener" thread,
> which is injected into the process by a lttng-ust constructor.
> What you will care about is how the tracepoint call-site (within a Xenomai
> thread) interacts with the ring buffers.
> The "default" setup for lttng-ust ring buffers is not suitable for Xenomai
> threads. The lttng-ust ring buffer is split into sub-buffers, each sub-buffer
> corresponding to a CTF trace "packet". When a sub-buffer is filled, lttng-ust
> invokes "write(2)" to a pipe to let the consumer daemon know there is data
> available in that ring buffer. You will want to get rid of that write(2) system
> call from a Xenomai thread.
> The proper configuration is to use lttng-enable-channel(1) "--read-timer"
> option (see https://lttng.org/docs/v2.11/#doc-channel-read-timer). This will
> ensure that the consumer daemon uses a polling approach to check periodically
> whether data needs to be consumed within each buffer, thus removing the
> use of the write(2) system call on the application-side.

Ah thanks.

But that's configuration outside of the RT app if I understand this correctly.
So if one configures a tracer wrong, then the app will suddenly misbehave.
Would be nice to be able to somehow tell that there is only read-timer allowed.

> > liburcu has configure options allow forcing the usage of this syscall
> > but not disabling it, which likely is necessary for Xenomai.
> I suspect what you'd need there is a way to allow a process to tell
> liburcu-bp (or liburcu) to always use the fall-back mechanism which does
> not rely on sys_membarrier. This could be allowed before the first use of
> the library. I think extending the liburcu APIs to allow this should be
> straightforward enough. This approach would be more flexible than requiring
> liburcu to be specialized at configure time. This new API would return an error
> if invoked with a liburcu library compiled with --disable-sys-membarrier-fallback.

I was under the impression, that you counted clock-cycles for every operation ;)
Not sure, maybe a separate lib for realtime is the better way. Having no option
can be considered foolproof, and sideeffects of the syscall not working would be
a real pain.

regards, Norbert

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