[lttng-dev] reading context fields causes syscalls

[Mathieu Desnoyers]

----- On Jul 14, 2022, at 9:10 AM, Norbert Lange nolange79 at gmail.com wrote:

> Hello Mathieu,
> 
> Am Do., 20. Mai 2021 um 16:16 Uhr schrieb Mathieu Desnoyers
> <mathieu.desnoyers at efficios.com>:
>>
>> ----- On May 20, 2021, at 9:42 AM, Norbert Lange nolange79 at gmail.com wrote:
>>
>> > Am Do., 20. Mai 2021 um 15:28 Uhr schrieb Mathieu Desnoyers
>> > <mathieu.desnoyers at efficios.com>:
>> >>
>> >> ----- On May 20, 2021, at 8:46 AM, Norbert Lange nolange79 at gmail.com wrote:
>> >>
>> >> > Am Mi., 19. Mai 2021 um 20:52 Uhr schrieb Mathieu Desnoyers
>> >> > <mathieu.desnoyers at efficios.com>:
>> >> >>
>> >> >> ----- On May 19, 2021, at 8:11 AM, lttng-dev lttng-dev at lists.lttng.org wrote:
>> >> >>
>> >> >> > Hello,
>> >> >> >
>> >> >> > Several context fields will cause a syscall atleast the first time a
>> >> >> > tracepoint is
>> >> >> > recorded. For example all of the following:
>> >> >> >
>> >> >> > `lttng add-context -c chan --userspace --type=vpid --type=vtid --type=procname`
>> >> >> >
>> >> >> > Each of them seems cached in TLS however, and most should never change
>> >> >> > after startup.
>> >> >> >
>> >> >> > As I am using Lttng over Xenomai, syscalls are strictly forbidden, I
>> >> >> > would like to have some function that prepares all data, which I can
>> >> >> > call on each thread before it switches to realtime work.
>> >> >> >
>> >> >> > Kinda similar to urcu_bp_register_thread, I'd like to have some
>> >> >> > `lttng_ust_warmup_thread` function that fetches the context values
>> >> >> > that can be cached. (urcu_bp_register_thread should be called there
>> >> >> > aswell)
>> >> >> > I considered just doing a tracepoint, but AFAIK the channel can be
>> >> >> > changed/configured after the process is running. So this is not robust
>> >> >> > enough.
>> >> >>
>> >> >> The new lttng_ust_init_thread() API in lttng-ust 2.13 would be the right
>> >> >> place to do this I think:
>> >> >>
>> >> >> /*
>> >> >>  * Initialize this thread's LTTng-UST data structures. There is
>> >> >>  * typically no need to call this, because LTTng-UST initializes its
>> >> >>  * per-thread data structures lazily, but it should be called explicitly
>> >> >>  * upon creation of each thread before signal handlers nesting over
>> >> >>  * those threads use LTTng-UST tracepoints.
>> >> >>  */
>> >> >>
>> >> >> It would make sense that this new initialization helper also initializes
>> >> >> all contexts which cache the result of a system call. Considering that
>> >> >> contexts can be used from the filter and capture bytecode interpreter, as
>> >> >> well as contexts added to channels, I think we'd need to simply initialize
>> >> >> them all.
> 
> Any update on that, is there an list of open points/features for lttng?

Hi Norbert,

You can find a feature wish list for the LTTng-UST project here:

  https://bugs.lttng.org/projects/lttng-ust

One option you have is to implement a patch and send it our way for review. We will
then review it as time/bandwidth allows.

However, if you would like the EfficiOS team to do the implementation, testing, and
integration, please contact us in private so we can discuss our consulting services.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com