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

[Mathieu Desnoyers]

----- On May 20, 2021, at 9:54 AM, lttng-dev lttng-dev at lists.lttng.org wrote:

> ----- On May 20, 2021, at 5:11 AM, lttng-dev lttng-dev at lists.lttng.org wrote:
> 
>> Am Do., 20. Mai 2021 um 10:28 Uhr schrieb MONTET Julien
>> <julien.montet at 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.

Thanks,

Mathieu

> a consistent value. With Xenomai it indeed appears to be prone to
> deadlock if a high priority Xenomai thread interrupts the kernel
> while the write seqlock is held, and then proceeds to loop forever
> on the read-side of the seqlock.
> 
> Note that for the in-kernel tracer clock read use-case, which
> needs to be able to happen from NMI context, I've contributed a
> modified version of the seqlock to the Linux kernel:
> 
> https://lwn.net/Articles/831540/ The seqcount latch lock type
> 
> It basically keeps two copies of the clock data structures, so the
> read-side never has to loop waiting for the updater: it simply gets
> redirected to the "stable" copy of the data.
> 
> The trade-off here is that with the latch lock used for clocks, a
> reader may observe time going slightly backwards between two clock
> reads when reading while specific clock rate adjustments are made
> by an updater. The clock user needs to be aware of this.
> 
> Thanks,
> 
> Mathieu
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com