[lttng-dev] Using lttng-ust with xenomai

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Fri Nov 22 14:03:30 EST 2019

----- On Nov 22, 2019, at 12:55 PM, Norbert Lange nolange79 at gmail.com wrote:

>> 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.

So an RT application would prohibit tracing to non-RT ring buffers ? IOW, if a
channel is configured without the --read-timer option, nothing would appear from
the RT threads in those buffers.

Should this be per-process or per-thread ?

>> > 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 ;)

Well it's just a new API that allows tweaking the state of a boolean which controls
branches which are already there on the fast-path. ;)

> 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.

e.g. a liburcu-bp-rt.so ? That would bring interesting integration challenges with
lttng-ust though. Should we then build a liblttng-ust-rt.so as well ?



Mathieu Desnoyers
EfficiOS Inc.

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