[lttng-dev] reading context fields causes syscalls

[Norbert Lange]

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.

regards, Norbert