[RFC] eal: per-thread constructors/destructors

[Morten Brørup]

This RFC introduces per-thread constructors/destructors:
* Per-thread constructors are functions called as the first thing from newly created threads. This can be used to initialize variables in thread-local storage, variables depending on the (tid_t) thread id, and to call setup functions that must be called from the thread itself.
* Per-thread destructors are functions called (from the thread) as the last thing when a thread ends.

At this time, I am seeking feedback on the concept and the proposed limitations.

Processes have __attribute__(constructor/destructor) to set up functions to be called before main(). Nothing similar exists for threads, so we have to design it ourselves.

The proposed per-thread constructors/destructors should not apply to all threads - only to threads created through the DPDK threads API. Agree?

DPDK has the RTE_INIT()/RTE_FINI() macros for adding process constructors/destructors at build time, so I propose a similar API, i.e. adding RTE_THREAD_INIT() and RTE_THREAD_FINI() macros to build a list of per-thread constructors and destructors at build time.

Two functions that call the list of per-thread constructors respectively destructors will be introduced.

The per-thread constructor function will be called from the newly created threads within DPDK:
1. EAL/SERVICE threads: In eal_thread_loop() before the loop.
2. CTRL threads: In ctrl_thread_init() before start_routine().
3. Non-EAL threads: In rte_thread_register().

Are any thread types missing in the list above?


The per-thread destructor function will also be registered by the per-thread constructor, using the POSIX pthread_cleanup_push() function.


Examples of where per-thread constructors are useful:
PRNG state initialization [1]: http://inbox.dpdk.org/dev/2a5121c7-369f-afde-0898-d45a5b368c3a@ericsson.com/
PMU event setup [2]: http://inbox.dpdk.org/dev/a059c403-8c6c-79c3-c3e9-a8c1815f5a14@ericsson.com/T/#m3d29fbc301059f007425ac148a4113e66d2ef983


Please also refer to the discussion [3] about lcore init callbacks for further thoughts. Especially the de-facto inability to establish constructors/destructors at runtime.

[3]: http://inbox.dpdk.org/dev/98CBD80474FA8B44BF855DF32C47DC35D875B6@smartserver.smartshare.dk/T/#m23b9a930a4050dc6b0305d3653754bd152c09ab7


Med venlig hilsen / Kind regards,
-Morten Brørup