[lttng-dev] Using lttng-ust with xenomai

Jan Kiszka jan.kiszka at siemens.com
Fri Nov 22 13:07:27 EST 2019

On 22.11.19 19:01, Norbert Lange wrote:
> Am Fr., 22. Nov. 2019 um 18:52 Uhr schrieb Jan Kiszka <jan.kiszka at siemens.com>:
>> On 22.11.19 18:44, Norbert Lange wrote:
>>> Am Fr., 22. Nov. 2019 um 16:52 Uhr schrieb Jan Kiszka <jan.kiszka at siemens.com>:
>>>> On 22.11.19 16:42, Mathieu Desnoyers wrote:
>>>>> ----- On Nov 22, 2019, at 4:14 AM, Norbert Lange nolange79 at gmail.com wrote:
>>>>>> Hello,
>>>>>> I already started a thread over at xenomai.org [1], but I guess its
>>>>>> more efficient to ask here aswell.
>>>>>> The basic concept is that xenomai thread run *below* Linux (threads
>>>>>> and irg handlers), which means that xenomai threads must not use any
>>>>> I guess you mean "irq handlers" here.
>>>>>> linux services like the futex syscall or socket communication.
>>>>>> ## tracepoints
>>>>>> expecting that tracepoints are the only thing that should be used from
>>>>>> the xenomai threads, is there anything using linux services.
>>>>>> the "bulletproof" urcu apparently does not need anything for the
>>>>>> reader lock (aslong as the thread is already registered),
>>>>> Indeed the first time the urcu-bp read-lock is encountered by a thread,
>>>>> the thread registration is performed, which requires locks, memory allocation,
>>>>> and so on. After that, the thread can use urcu-bp read-side lock without
>>>>> requiring any system call.
>>>> So, we will probably want to perform such a registration unconditionally
>>>> (in case lttng usage is enabled) for our RT threads during their setup.
>>> Who is we? Do you plan to add automatic support at xenomai mainline?
>>> But yes, some setup is likely needed if one wants to use lttng
>> I wouldn't refuse patches to make this happen in mainline. If patches
>> are best applied there. We could use a deterministic and fast
>> application tracing frame work people can build upon, and that they can
>> smoothly combine with system level traces.
> Sure (good to hear), I just dont think enabling it automatic/unconditionally
> is a good thing.

I don't disagree. If it requires built-time control or could also be 
enabled during application setup is something to be seen later.

>>>>>> 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.
>>>>> If you have control over your entire system's kernel, you may want to try
>>>>> just configuring the kernel within CONFIG_MEMBARRIER=n in the meantime.
>>>>> Another thing to make sure is to have a glibc and Linux kernel which perform
>>>>> clock_gettime() as vDSO for the monotonic clock, because you don't want a
>>>>> system call there. If that does not work for you, you can alternatively
>>>>> implement your own lttng-ust and lttng-modules clock plugin .so/.ko to override
>>>>> the clock used by lttng, and for instance use TSC directly. See for instance
>>>>> the lttng-ust(3) LTTNG_UST_CLOCK_PLUGIN environment variable.
>>>> clock_gettime & Co for a Xenomai application is syscall-free as well.
>>> Yes, and that gave me a deadlock already, if a library us not compiled
>>> for Xenomai,
>>> it will either use the syscall (and you detect that immediatly) or it
>>> will work most of the time,
>>> and lock up once in a while if a Linux thread took the "writer lock"
>>> of the VDSO structures
>>> and your high priority xenomai thread is busy waiting infinitely.
>>> Only sane approach would be to use either the xenomai function directly,
>>> or recreate the function (rdtsc + interpolation on x86).
>> rdtsc is not portable, thus a no-go.
> Its not portable, but you have equivalents on ARM, powerpc.
> ie. "Do the same think as Xenomai"

If you use existing code, I'm fine. Just not invent something "new" here.

>>> Either compiling/patching lttng for Cobalt (which I really would not
>>> want to do) or using a
>>> clock plugin.
>> I suspect you will want to have at least a plugin that was built against
>> Xenomai libs.
> That will then do alot other stuff like spwaning a printf thread.
>>> If the later is supposed to be minimal, then that would mean I would
>>> have to get the
>>> interpolation factors cobalt uses (without bringing in libcobalt).
>>> Btw. the Xenomai and Linux monotonic clocks arent synchronised at all
>>> AFAIK, so timestamps will
>>> be different to the rest of Linux.
>> CLOCK_HOST_REALTIME is synchronized.
> Thats not monotonic?

Yeah, it's REALTIME, in synch with CLOCK_REALTIME of Linux. 
CLOCK_MONOTONIC should have a static offset at worst. I think that could 
be resolved if it wasn't yet.

>>> On my last plattform I did some tracing using internal stamp and
>>> regulary wrote a
>>> block with internal and external timestamps so those could be
>>> converted "offline".
>> Sounds not like something we want to promote.
> This was a questing to lttng and its tool environment. I suppose we
> werent the first
> ones with multiple clocks in a system.
> If anything needs to be done in Xenomai it might be a concurrent
> readout of Linux/cobalt time(s),
> the rest would be done offline, potentially on another system.

Sure, doable, but I prefer not having to do that.


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