From: Norbert Lange <email@example.com>
To: Mathieu Desnoyers <firstname.lastname@example.org>
Cc: Jan Kiszka <email@example.com>,
paulmck <firstname.lastname@example.org>, Xenomai <email@example.com>
Subject: Re: Using lttng-ust with xenomai
Date: Fri, 22 Nov 2019 20:57:57 +0100 [thread overview]
Message-ID: <CADYdroM7acqfMym1sUbwaa773SLSzHPSni9uRxiLZbbHtteLug__24426.firstname.lastname@example.org> (raw)
Am Fr., 22. Nov. 2019 um 20:00 Uhr schrieb Mathieu Desnoyers
> ----- On Nov 22, 2019, at 12:44 PM, Norbert Lange email@example.com wrote:
> > Am Fr., 22. Nov. 2019 um 16:52 Uhr schrieb Jan Kiszka <firstname.lastname@example.org>:
> >> On 22.11.19 16:42, Mathieu Desnoyers wrote:
> >> >
> >> > That's indeed a good point. I suspect membarrier may not send any IPI
> >> > to Xenomai threads (that would have to be confirmed). I suspect the
> >> > latency introduced by this IPI would be unwanted.
> >> Is an "IPI" a POSIX signal here? Or are real IPI that delivers an
> >> interrupt to Linux on another CPU? The latter would still be possible,
> >> but it would be delayed until all Xenomai threads on that core eventual
> >> took a break (which should happen a couple of times per second under
> >> normal conditions - 100% RT load is an illegal application state).
> > Not POSIX, some inter-thread interrupts. point is the syscall waits
> > for the set of
> > registered *running* Linux threads.
> Just a small clarification: the PRIVATE membarrier command does not *wait*
> for other threads, but it rather ensures that all other running threads
> have had IPIs that issue memory barriers before it returns.
Ok, normal linux IRQs have to wait till Xenomai gives the cores back,
hence the waiting.
> >> >
> >> > 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).
> > Either compiling/patching lttng for Cobalt (which I really would not
> > want to do) or using a
> > clock plugin.
> > 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.
> > 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".
> > Anything similar with lttng or tools handling the traces?
> Can a Xenomai thread issue clock_gettime(CLOCK_MONOTONIC) ?
Yes it can, if the calls goes through the VDSO, then it mostly works.
And once in a while deadlocks the system if a Xenomai thread waits for a
spinlock that the Linux kernel owns and doesnt give back as said thread will
not let the Linux Kernel run (as described above).
> AFAIK we don't have tooling to do what you describe out of the box,
> but it could probably be implemented as a babeltrace 2 filter plugin.
There are alot ways to do that, I hoped for some standardized way.
next prev parent reply other threads:[~2019-11-22 19:58 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.email@example.com>
2019-11-22 15:52 ` Jan Kiszka
[not found] ` <firstname.lastname@example.org>
2019-11-22 17:01 ` Mathieu Desnoyers
[not found] ` <480743920.929.1574442078799.JavaMail.email@example.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] ` <firstname.lastname@example.org>
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] ` <email@example.com>
2019-11-22 21:38 ` Norbert Lange
2019-11-22 19:00 ` Mathieu Desnoyers
[not found] ` <2088324778.1063.1574449205629.JavaMail.firstname.lastname@example.org>
2019-11-22 19:57 ` Norbert Lange [this message]
[not found] ` <CADYdroM7acqfMym1sUbwaa773SLSzHPSni9uRxiLZbbHtteLug@mail.gmail.com>
2019-11-22 20:15 ` Mathieu Desnoyers
[not found] ` <547908110.1420.1574453756850.JavaMail.email@example.com>
2019-11-22 21:27 ` [lttng-dev] " Norbert Lange via Xenomai
2019-11-22 17:55 ` Norbert Lange
[not found] ` <CADYdroNGcY6adA5cGTzwzsKqO7+iuWts9k8Haz8k3HSvzQfcfirstname.lastname@example.org>
2019-11-22 19:03 ` Mathieu Desnoyers
[not found] ` <1007669875.1091.1574449410739.JavaMail.email@example.com>
2019-11-22 20:04 ` Norbert Lange
2019-11-22 9:14 Norbert Lange
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