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From: Norbert Lange via lttng-dev <lttng-dev@lists.lttng.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: MONTET Julien <julien.montet@reseau.eseo.fr>,
	lttng-dev <lttng-dev@lists.lttng.org>,
	 Jan Kiszka <jan.kiszka@siemens.com>,
	Xenomai <xenomai@xenomai.org>
Subject: Re: [lttng-dev] LTTng - Xenomai : different results between timestamp-lttng and rt_time_read()
Date: Thu, 20 May 2021 17:39:30 +0200	[thread overview]
Message-ID: <CADYdroOCUu7BWqMZ1CMa5mHSJB50V1aS+98sVfgXQRPdnJTyYg@mail.gmail.com> (raw)
In-Reply-To: <186729016.52398.1621523346736.JavaMail.zimbra@efficios.com>

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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  parent reply	other threads:[~2021-05-20 15:39 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-20  7:58 MONTET Julien via lttng-dev
2021-05-20  8:20 ` Norbert Lange via lttng-dev
2021-05-20  8:28   ` MONTET Julien via lttng-dev
2021-05-20  9:11     ` Norbert Lange via lttng-dev
2021-05-20 13:54       ` Mathieu Desnoyers via lttng-dev
2021-05-20 13:56         ` Mathieu Desnoyers via lttng-dev
2021-05-20 15:09           ` Mathieu Desnoyers via lttng-dev
2021-05-20 15:34             ` Jan Kiszka via lttng-dev
2021-05-20 15:39             ` Norbert Lange via lttng-dev [this message]
2021-05-21 10:13               ` MONTET Julien via lttng-dev
2021-05-25  8:46                 ` Norbert Lange via lttng-dev

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