Jan Kiszka wrote:
Philippe Gerum wrote:

Gilles Chanteperdrix wrote:

Jan Kiszka wrote:
> (...)As Xenomai does not support hard-RT signal delivery yet (...)

This is the next feature missing to the POSIX skin. I would like to
implement this, but I am not sure which way to go :
- either, if it is possible, getting Linux signals services to run in
 every domain at Adeos level, by replacing spinlocks with spinlocks_hw
 and such kind of tricks;

Would be a nightmare, I think. Way too many paths are involved in the
vanilla kernel, and this would be overkill wrt what we want to do.
Actually, what we need is basically exposing the nucleus signal
interface to user-space, and map Linux RT signals over nucleus signals.
Other (non-RT) Linux signals would keep on being handled in secondary
mode the way they are now.


- or adding a generic service at the adeos layer (a hook called when
 returning to user-space), building a generic user-space signals
 service at the nucleus level, and finally building all posix signals
 services on top of this.

A (maybe easier) third option would be to generalize some kind of
pseudo-asynchronous call support, with a worker thread operating on a
dedicated priority level inside applications registering for
asynchronous notifications. The kernel-side would handle the server
wakeups, providing a unified interface for pending on hooks, signals,
watchdogs etc. It would also need to properly multiplex those events
notified from within the skins, and wake up the right pending server in
user-space, which would in turn fire the user provided handler, all in
primary mode. In any case, this would not be more costly latency-wise
than implementing mere callouts, since most of the switching cost comes
from the MMU switch, which we would have to do in both cases, anyway.


We would need a "shadow" priority level for each real one so that those
handlers do not cause any priority inversions (the main RT issue of
servers).

Not if the worker threads inherit the priority of the target threads dynamically.

 Moreover, it would require a bulk of extra threads, actually
one per used prio level, to handle all those calls with the correct
priority. My feeling: too costly, memory-wise.

You would just need one worker thread per-application and per-cpu. Async calls are always serialized, at least on a given CPU. The idea is to provide some event multiplexing capabilities to the kernel-side, instead of implementing a worker for each and every pended event.

The only thing that would differ from a real signal handling is the actual backtrace of the handler.

Another option without worker thread would be to implement the same kind of pending event detection the kernel does on the return path from Xenomai syscalls using some reserved CPU register, and ask the kernel side about the callout to fire and the argument to pass the handler whenever some pending event is detected.


Jan



--

Philippe.

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