Re: [lttng-dev] LTTng - Xenomai : different results between timestamp-lttng and rt_time_read()

[Norbert Lange via lttng-dev <lttng-dev@lists.lttng.org>]

Am Do., 20. Mai 2021 um 17:09 Uhr schrieb Mathieu Desnoyers
<mathieu.desnoyers@efficios.com>:
>
> ----- On May 20, 2021, at 9:56 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:
>
> > ----- On May 20, 2021, at 9:54 AM, lttng-dev lttng-dev@lists.lttng.org wrote:
> >
> >> ----- On May 20, 2021, at 5:11 AM, lttng-dev lttng-dev@lists.lttng.org wrote:
> >>
> >>> Am Do., 20. Mai 2021 um 10:28 Uhr schrieb MONTET Julien
> >>> <julien.montet@reseau.eseo.fr>:
> >>>>
> >>>> Hi Norbert,
> >>>>
> >>>> Thank you for your answer !
> >>>>
> >>>> Yes, I am using a Xenomai cobalt - xenomai is 3.1
> >>>> cat /proc/xenomai/version => 3.1
> >>>>
> >>>> After the installation, I tested "test tools" in /proc/xenomai/ and it worked
> >>>> nice.
> >>>
> >>> Just asked to make sure, thought the scripts usual add some -xeno tag
> >>> to the kernel version.
> >>>
> >>>> What do you mean by "it might deadlock really good" ?
> >>>
> >>> clock_gettime will either use a syscall (kills realtime always) or is
> >>> optimized via VDSO (which very likely is your case).
> >>>
> >>> What happens is that the kernel will take a spinlock, then write new
> >>> values, then releases the spinlock.
> >>> your program will aswell spin (but just to see if the spinlock is
> >>> free), read the values and interpolates them.
> >>>
> >>> But if your program interrupts the kernel while the kernel holds the
> >>> lock (all on the same cpu core), then it will spin forever and the
> >>> kernel will never execute.
> >>
> >> Just one clarification: the specific locking strategy used by the
> >> Linux kernel monotonic clock vDSO is a "seqlock", where the kernel
> >> sets a bit which keeps concurrent readers looping until they observe
> >
> > When I say "sets a bit", I actually mean "increment a sequence counter",
> > and readers observe either odd or even state, thus knowing whether
> > they need to retry, and whether the value read before/after reading
> > the data structure changed.
>
> Looking again at the Linux kernel's kernel/time/vsyscall.c implementation
> of vdso_update_{begin,end}, I notice that interrupts are disabled across
> the entire update. So I understand that the Interrupt pipeline (I-pipe)
> interrupt gets delivered even when the kernel disables interrupts. Did
> you consider modifying the I-pipe kernel patch to change the vdso update so
> it updates the vdso from within an I-pipe virq handler ?

Yes, I did use an non-upstreamed patch for a while to get things in order:
https://www.xenomai.org/pipermail/xenomai/2018-December/040134.html

I would prefer just a NMI safe source that might jump back a bit, no matter how.

> AFAIU this would allow Xenomai userspace to use the Linux kernel vDSO
> clock sources.

The Xenomai folks are trying to get their next-gen abstraction "dovetail" closer
coupled to the kernel, AFAIR their will be VDSO support and
unification of the clock sources.

Still need to get stuff running today =)

Norbert
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