Re: [Xenomai-core] [PATCH] Mayday support

[Philippe Gerum <rpm@xenomai.org>]

On Fri, 2010-08-20 at 14:32 +0200, Jan Kiszka wrote:
> Jan Kiszka wrote:
> > Philippe Gerum wrote:
> >> I've toyed a bit to find a generic approach for the nucleus to regain
> >> complete control over a userland application running in a syscall-less
> >> loop.
> >>
> >> The original issue was about recovering gracefully from a runaway
> >> situation detected by the nucleus watchdog, where a thread would spin in
> >> primary mode without issuing any syscall, but this would also apply for
> >> real-time signals pending for such a thread. Currently, Xenomai rt
> >> signals cannot preempt syscall-less code running in primary mode either.
> >>
> >> The major difference between the previous approaches we discussed about
> >> and this one, is the fact that we now force the runaway thread to run a
> >> piece of valid code that calls into the nucleus. We do not force the
> >> thread to run faulty code or at a faulty address anymore. Therefore, we
> >> can reuse this feature to improve the rt signal management, without
> >> having to forge yet-another signal stack frame for this.
> >>
> >> The code introduced only fixes the watchdog related issue, but also does
> >> some groundwork for enhancing the rt signal support later. The
> >> implementation details can be found here:
> >> http://git.xenomai.org/?p=xenomai-rpm.git;a=commit;h=4cf21a2ae58354819da6475ae869b96c2defda0c
> >>
> >> The current mayday support is only available for powerpc and x86 for
> >> now, more will come in the next days. To have it enabled, you have to
> >> upgrade your I-pipe patch to 2.6.32.15-2.7-00 or 2.6.34-2.7-00 for x86,
> >> 2.6.33.5-2.10-01 or 2.6.34-2.10-00 for powerpc. That feature relies on a
> >> new interface available from those latest patches.
> >>
> >> The current implementation does not break the 2.5.x ABI on purpose, so
> >> we could merge it into the stable branch.
> >>
> >> We definitely need user feedback on this. Typically, does arming the
> >> nucleus watchdog with that patch support in, properly recovers from your
> >> favorite "get me out of here" situation? TIA,
> >>
> >> You can pull this stuff from
> >> git://git.xenomai.org/xenomai-rpm.git, queue/mayday branch.
> >>
> > 
> > I've retested the feature as it's now in master, and it has one
> > remaining problem: If you run the cpu hog under gdb control and try to
> > break out of the while(1) loop, this doesn't work before the watchdog
> > expired - of course. But if you send the break before the expiry (or hit
> > a breakpoint), something goes wrong. The Xenomai task continues to spin,
> > and there is no chance to kill its process (only gdb).
> > 
> > # cat /proc/xenomai/sched
> > CPU  PID    CLASS  PRI      TIMEOUT   TIMEBASE   STAT       NAME
> >   0  0      idle    -1      -         master     RR         ROOT/0

Eeek, we really need to have a look at this funky STAT output.

> >   1  0      idle    -1      -         master     R          ROOT/1
> >   0  6120   rt      99      -         master     Tt         cpu-hog
> > # cat /proc/xenomai/stat
> > CPU  PID    MSW        CSW        PF    STAT       %CPU  NAME
> >   0  0      0          0          0     00500088    0.0  ROOT/0
> >   1  0      0          0          0     00500080   99.7  ROOT/1
> >   0  6120   0          1          0     00342180  100.0  cpu-hog
> >   0  0      0          21005      0     00000000    0.0  IRQ3340: [timer]
> >   1  0      0          35887      0     00000000    0.3  IRQ3340: [timer]
> > 
> 
> Fixable by this tiny change:
> 
> diff --git a/ksrc/nucleus/sched.c b/ksrc/nucleus/sched.c
> index 5242d9f..04a344e 100644
> --- a/ksrc/nucleus/sched.c
> +++ b/ksrc/nucleus/sched.c
> @@ -175,7 +175,8 @@ void xnsched_init(struct xnsched *sched, int cpu)
>  			     xnthread_name(&sched->rootcb));
>  
>  #ifdef CONFIG_XENO_OPT_WATCHDOG
> -	xntimer_init(&sched->wdtimer, &nktbase, xnsched_watchdog_handler);
> +	xntimer_init_noblock(&sched->wdtimer, &nktbase,
> +			     xnsched_watchdog_handler);
>  	xntimer_set_name(&sched->wdtimer, "[watchdog]");
>  	xntimer_set_priority(&sched->wdtimer, XNTIMER_LOPRIO);
>  	xntimer_set_sched(&sched->wdtimer, sched);
> 
> 
> I.e. the watchdog timer should not be stopped by any ongoing debug
> session of a Xenomai app. Will queue this for upstream.

Yes, that makes a lot of sense now. The watchdog would not fire if the
task was single-stepped anyway, since the latter would have been moved
to secondary mode first.

Did you see this bug happening in a uniprocessor context as well?

> 
> Jan
> 

-- 
Philippe.