Re: [Xenomai-core] High latencies on ARM.

[Philippe Gerum <rpm@xenomai.org>]

Gilles Chanteperdrix wrote:
> Philippe Gerum wrote:
>  > Gilles Chanteperdrix wrote:
>  > > Philippe Gerum wrote:
>  > >  > Gilles Chanteperdrix wrote:
>  > >  > > On Jan 23, 2008 7:34 PM, Philippe Gerum <rpm@xenomai.org> wrote:
>  > >  > >> Gilles Chanteperdrix wrote:
>  > >  > >>> On Jan 23, 2008 6:48 PM, Philippe Gerum <rpm@xenomai.org> wrote:
>  > >  > >>>> Gilles Chanteperdrix wrote:
>  > >  > >>>>> Gilles Chanteperdrix wrote:
>  > >  > >>>>>  > Please find attached a patch implementing these ideas. This adds some
>  > >  > >>>>>  > clutter, which I would be happy to reduce. Better ideas are welcome.
>  > >  > >>>>>  >
>  > >  > >>>>>
>  > >  > >>>>> Ok. New version of the patch, this time split in two parts, should
>  > >  > >>>>> hopefully make it more readable.
>  > >  > >>>>>
>  > >  > >>>> Ack. I'd suggest the following:
>  > >  > >>>>
>  > >  > >>>> - let's have a rate limiter when walking the zombie queue in
>  > >  > >>>> __xnpod_finalize_zombies. We hold the superlock here, and what the patch
>  > >  > >>>> also introduces is the potential for flushing more than a single TCB at
>  > >  > >>>> a time, which might not always be a cheap operation, depending on which
>  > >  > >>>> cra^H^Hode runs on behalf of the deletion hooks for instance. We may
>  > >  > >>>> take for granted that no sane code would continuously create more
>  > >  > >>>> threads than we would be able to finalize in a given time frame anyway.
>  > >  > >>> The maximum number of zombies in the queue is
>  > >  > >>> 1 + XNARCH_WANT_UNLOCKED_CTXSW, since a zombie is added to the queue
>  > >  > >>> only if a deleted thread is xnpod_current_thread(), or if the XNLOCKSW
>  > >  > >>> bit is armed.
>  > >  > >> Ack. rate_limit = 1? I'm really reluctant to increase the WCET here,
>  > >  > >> thread deletion isn't cheap already.
>  > >  > > 
>  > >  > > I am not sure that holding the nklock while we run the thread deletion
>  > >  > > hooks is really needed.
>  > >  > > 
>  > >  > 
>  > >  > Deletion hooks may currently rely on the following assumptions when running:
>  > >  > 
>  > >  > - rescheduling is locked
>  > >  > - nklock is held, interrupts are off
>  > >  > - they run on behalf of the deletor context
>  > >  > 
>  > >  > The self-delete refactoring currently kills #3 because we now run the
>  > >  > hooks after the context switch, and would also kill #2 if we did not
>  > >  > hold the nklock (btw, enabling the nucleus debug while running with this
>  > >  > patch should raise an abort, from xnshadow_unmap, due to the second
>  > >  > assertion).
>  > >  > 
>  > 
>  > Forget about this; shadows are always exited in secondary mode, so
>  > that's fine, i.e. xnpod_current_thread() != deleted thread, hence we
>  > should always run the deletion hooks immediately on behalf of the caller.
> 
> What happens if the watchdog kills a user-space thread which is
> currently running in primary mode ? If I read xnpod_delete_thread
> correctly, the SIGKILL signal is sent to the target thread only if it is
> not the current thread.
> 

I'd say: zombie queuing from xnpod_delete, then shadow unmap on behalf
of the next switched context which would trigger the lo-stage unmap
request -> wake_up_process against the Linux side and asbestos underwear
provided by the relax epilogue, which would eventually reap the guy
through do_exit(). As a matter of fact, we would still have the
unmap-over-non-current issue, that's true.

Ok, could we try coding a damn Tetris instead? Pong, maybe? Gasp...

-- 
Philippe.