Jan Kiszka wrote:
Philippe Gerum wrote:

Given that, a shadow thread which remains in secondary mode is seen as
suspended by the Xenomai scheduler, which means that the additional
overhead is close to nil (aside of an additional shadow tcb), until it
tries to grab the resource and thus moves to the primary domain, just
for going to sleep.

I'm not concerned about a passive shadow, my aim is to reduce the impact
of active shadow threads without strict timing requirements. When there
is a larger number of them, they should not influence RT scheduling
significantly for just being woken up.

This issue already exists now, since interrupts are off when Linux performs a context switch, so that we can't wreck the whole thing by allowing a shadow to preempt it in the middle of the mm switch. Since non-RT shadow would run at priority level 0, my feeling is that it would not change the current situation, except perhaps on SMP where the nklock would be held a little more.

The other point might be the code path length of both non-RT wakeup
suspend/wakeup mechanisms. It's only an assessment, but I think this way
would be shorter. Anyway, numbers would speak clearer, and I guess this
should be done first before any further design decision.


IOW, I would rather use the allow-non-SCHED_FIFO-shadow patch to allow
regular Linux tasks to pend seamlessly on Xenomai sync objects as true
shadows, and add to the rtdm blocking syscall handler some code that
would transparently promote any calling regular task as a Xenomai shadow
on the fly, before undergoing the request. The other advantage of such
approach is that the non-RT pending task is still known from the Xenomai
sub-system (/proc/xenomai/ interface and so on), which may be useful
given that it interacts with it. You could still apply a FIFO pending
order with mixed RT and non-RT tasks the same way.

Hmm, on-demand shadowing might be an option. Ok, once non-SCHED_FIFO is
merged, we should have a look at both approaches from the performance
POV and compare potential differences with the required additional code.

Moving RTDM to native preemption would also be simpler, since RTDM would
expect the issue to be handled at core level directly, and in the
preempt-rt case, the issue of the caller's domain is actually a non-issue.

Well, this is already the case for my proposal: no special handling of
rtdm_event_* depending on the caller's context anymore.

But still, you have to change RTDM's code do implement that, more than you would have to do in promoting a regular task as a non-RT shadow.




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