> > 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 _______________________________________________ lttng-dev mailing list lttng-dev@lists.lttng.org https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
