[Xenomai] copperplate/registry daemon connection failure

Philippe Gerum rpm at xenomai.org
Fri Jan 6 12:00:27 CET 2017

On 01/06/2017 10:54 AM, Ronny Meeus wrote:
> On Fri, Jan 6, 2017 at 10:29 AM, Philippe Gerum <rpm at xenomai.org> wrote:
>> On 01/06/2017 10:21 AM, Ronny Meeus wrote:
>>> That logic I had seen before.
>>> As I understand the code it just tries 3 times to connect to the daemon
>>> and if not successful, it just tries to start it again and reconnects ...
>>> I find it a strange logic just to try something 3 times and hope it
>>> will succeed.
>>> In our case the CPU is fully loaded with RT threads so my assumption is that
>>> the daemon, running at nonRT prio will not be scheduled at all.
>>> (also see the traces above that confirm my assumptions)
>>> I would expect to see some kind of synchronization mechanism between the
>>> daemon and the application.
>> The point is that such init code is aimed at running early, prior to any
>> Xenomai application code, this is cold bootstrap code. The fact that
>> your app can spawn threads overconsuming the CPU earlier than Xenomai's
>> basic init code is a problem for Xenomai.
> Philippe,
> on our system we have a lot of Xenomai applications running, it can be up to 10
> or more. So it is impossible to guarantee that there will be CPU power available
> at the moment Xenomai init is called.
> Next to the application code also the Linux kernel threads can consume a lot of
> CPU power (especially during init).

I don't see how your app could ever compete with drivers during the
kernel bootstrap phase, just because no application can run until user
mode is started, which is last in the process, by definition.

If referring to kernel helper threads overconsuming CPU during plain
runtime or soon after user mode is entered, maybe you should consider
determining why this happens, this does not look quite normal
(vendor-originated mmc driver with broken power mgmt, massive logging on
slow flash medium?). Maybe you did already, and I would be interested to
know about your findings.

> Xenomai applications can be started during init but also at runtime, so it is
> impossible to make assumptions about the availability of CPU power.

You obviously do make assumptions about the CPU power, such as assuming
that your system can cope in a deterministic way with running distinct
or even unrelated set of CPU-hungry threads from multiple real-time apps
concurrently. Xenomai makes the assumption that the current CPU should
be able to process all of the pending regular (non-rt) activity within 3
seconds, which seems reasonable. We could make it 30, no issue with
that, but that would not address the real problem anyway.

Your point is about requiring Xenomai to work around a seemingly massive
overload condition in the regular Linux system when the app initializes,
hoping for the best. I don't think this is the way to go, this would
only paper over the core issue, with potentially nasty effects.

Typically, a consequence of raising the priority of registry threads to
address this issue would be to serve fuse-fs requests at high (rt)
priority, directly competing with other SCHED_RR/SCHED_FIFO threads in
the system, since you don't run Cobalt, and therefore the co-kernel
could not save your day in this case.

Therefore, anyone issuing "cat /var/run/xenomai/*/*" on a terminal would
actually compete with some real-time threads in your application(s),
possibly delaying them for an undefined amount of time. At any rate, our
fuse-fs threads that do string formatting to output human-readable
reports upon (interactive) request should not compete with real-time
threads, really.

Regarding the fact that your system cannot respond within 3 seconds to a
socket connection, you still have the option to start the daemon
separately, before the application is launched. Any showstopper with
that option?


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