demonstrator: take on shared event handler threads

[haukepetersen]

Contribution description

Quickly drafted out my own ideas about the topic discussed in #12459. This is not meant to be ever merged, its rather to demonstrate some thoughts and contribute to the discussion pointed out above!

I wanted to try out, how the use of the term tasklet would feel like when just just to provided the shared event handler threads (as proposed in #12474). If the discussion in the end just boils down to the name of the submodule, I couldn't care less. But the main things to take are IMHO:

• negligible overhead compared to using event_x() API call directly (in this case one array lookup)
• being able to use all functionality that event provides -> timer handling ('event/timeout'), canceling events, ...

Testing procedure

never to be tested :-)

Issues/PRs references

#12459 and #12474

[haukepetersen]

Forgot: the discussion takes place in #12459, so please comment there.

[maribu]

You could do something like this:

#define TASKLET_PRIO_LOWEST (0U)

#ifdef MODULE_TASKLET_PRIO_LOW
#define TASKLET_PRIO_LOW (TASKLET_PRIO_LOWEST + 1)
#else
#define TASKLET_PRIO_LOW (TASKLET_PRIO_LOWEST)
#endif

#ifdef MODULE_TASKLET_PRIO_MEDIUM
#define TASKLET_PRIO_MEDIUM (TASKLET_PRIO_LOW + 1)
#else
#define TASKLET_PRIO_LOW (TASKLET_PRIO_LOW)
#endif

#ifdef MODULE_TASKLET_PRIO_HIGH
#define TASKLET_PRIO_HIGH (TASKLET_PRIO_MEDIUM + 1)
#else
#define TASKLET_PRIO_HIGH (TASKLET_PRIO_MEDIUM)
#endif

#ifdef MODULE_TASKLET_PRIO_HIGHEST
#define TASKLET_PRIO_HIGHEST (TASKLET_PRIO_HIGH + 1)
#else
#define TASKLET_PRIO_HIGHEST (TASKLET_PRIO_HIGH)
#endif

#define TASKLET_PRIO_NUMOF (TASKLET_PRIO_HIGHEST + 1)

One could argue that it is a bit confusing that the defines for all 5 priority levels are always defined. But I would say that this is actually an advantage, as I could decided later on that I do not need the divide in lowest and low without having to change all uses of TASKLET_PRIO_LOW to TASKLET_PRIO_LOWEST.

[haukepetersen]

As stated, please lets first focus on the overall concept before jumping into implementation details...

[haukepetersen]

Just so nobody gets a wrong idea: defining static variables in a header file, as done in this brain dump code, is a pretty stupid think of course :-)

[jia200x]

I think having multiple queues for now is a good approach. Eventually everything could be represented with a couple of queues, but this gives as a lot of flexibility for now.
One could think of:

#ifndef NET_RX_PRIO
#define NET_RX_PRIO TASKLET_PRIO_HIGH
#endif

#ifndef FOO_PRIO
#define FOO_PRIO TASKLET_PRIO_LOW
#endif

So, this gives the developer the mechanism to adapt the OS depending of the real time requirements.

[haukepetersen]

Thats probably the direction to go.

Closing this in favor of #12474, so lets see about the implementation specifics there.