From: Norbert Lange <nolange79@gmail.com>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>,
lttng-dev <lttng-dev@lists.lttng.org>,
paulmck <paulmck@kernel.org>, Xenomai <xenomai@xenomai.org>
Subject: Re: Using lttng-ust with xenomai
Date: Fri, 22 Nov 2019 18:55:38 +0100 [thread overview]
Message-ID: <CADYdroNGcY6adA5cGTzwzsKqO7+iuWts9k8Haz8k3HSvzQfc=g__36699.4488447503$1574445386$gmane$org@mail.gmail.com> (raw)
In-Reply-To: <2012667816.853.1574437363737.JavaMail.zimbra@efficios.com>
>
> 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
next prev parent reply other threads:[~2019-11-22 17:55 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CADYdroN-i9yrpd-wjPSW36GUptRV+kOCJT=Yv6+Z5sCVBmo_SQ@mail.gmail.com>
2019-11-22 15:42 ` Using lttng-ust with xenomai Mathieu Desnoyers
[not found] ` <2012667816.853.1574437363737.JavaMail.zimbra@efficios.com>
2019-11-22 15:52 ` Jan Kiszka
[not found] ` <4aab99be-5451-4582-f75d-7637614b1d37@siemens.com>
2019-11-22 17:01 ` Mathieu Desnoyers
[not found] ` <480743920.929.1574442078799.JavaMail.zimbra@efficios.com>
2019-11-22 17:36 ` Jan Kiszka
2019-11-22 17:44 ` Norbert Lange
[not found] ` <CADYdroOh+T8pOcNBW74KSMfCh--ujD8L3_G96LWR1migpsUq0g@mail.gmail.com>
2019-11-22 17:52 ` Jan Kiszka
[not found] ` <63a51fa5-db96-9cf9-0eb3-51954ebf98f4@siemens.com>
2019-11-22 18:01 ` Norbert Lange
[not found] ` <CADYdroP+H3DiqCaH9o1jHxAurh_2YG_p7MK4H2kFqSoTCV0w6A@mail.gmail.com>
2019-11-22 18:07 ` Jan Kiszka
[not found] ` <a2036ba8-c22a-e549-b68d-35524ee7f9a9@siemens.com>
2019-11-22 21:38 ` Norbert Lange
2019-11-22 19:00 ` Mathieu Desnoyers
[not found] ` <2088324778.1063.1574449205629.JavaMail.zimbra@efficios.com>
2019-11-22 19:57 ` Norbert Lange
[not found] ` <CADYdroM7acqfMym1sUbwaa773SLSzHPSni9uRxiLZbbHtteLug@mail.gmail.com>
2019-11-22 20:15 ` Mathieu Desnoyers
[not found] ` <547908110.1420.1574453756850.JavaMail.zimbra@efficios.com>
2019-11-22 21:27 ` [lttng-dev] " Norbert Lange via Xenomai
2019-11-22 17:55 ` Norbert Lange [this message]
[not found] ` <CADYdroNGcY6adA5cGTzwzsKqO7+iuWts9k8Haz8k3HSvzQfc=g@mail.gmail.com>
2019-11-22 19:03 ` Mathieu Desnoyers
[not found] ` <1007669875.1091.1574449410739.JavaMail.zimbra@efficios.com>
2019-11-22 20:04 ` Norbert Lange
2019-11-22 9:14 Norbert Lange
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