Re: Stop scheduler on panic

Alexander Motin <mav_at_FreeBSD.org> wrote:

> On 11/17/11 13:14, Kostik Belousov wrote:
> > On Thu, Nov 17, 2011 at 11:29:02AM +0200, Alexander Motin wrote:
> >> On 11/17/11 11:06, Kostik Belousov wrote:
> >>> On Thu, Nov 17, 2011 at 10:40:58AM +0200, Alexander Motin wrote:
> >>>> On 11/17/11 10:15, Kostik Belousov wrote:
> >>>>> On Thu, Nov 17, 2011 at 01:07:38AM +0200, Alexander Motin wrote:
> >>>>>> On 17.11.2011 00:21, Andriy Gapon wrote:
> >>>>>>> on 16/11/2011 21:27 Fabian Keil said the following:
> >>>>>>>> Kostik Belousov<kostikbel_at_gmail.com>  wrote:
> >>>>>>>>
> >>>>>>>>> I was tricked into finishing the work by Andrey Gapon, who developed
> >>>>>>>>> the patch to reliably stop other processors on panic.  The patch
> >>>>>>>>> greatly improves the chances of getting dump on panic on SMP host.
> >>>>>>>>
> >>>>>>>> I tested the patch trying to get a dump (from the debugger) for
> >>>>>>>> kern/162036, which currently results in the double fault reported in:
> >>>>>>>> http://lists.freebsd.org/pipermail/freebsd-current/2011-September/027766.html
> >>>>>>>>
> >>>>>>>> It didn't help, but also didn't make anything worse.

> >>>>>>> The mi_switch recursion looks very familiar to me:
> >>>>>>> mi_switch() at mi_switch+0x270
> >>>>>>> critical_exit() at critical_exit+0x9b
> >>>>>>> spinlock_exit() at spinlock_exit+0x17
> >>>>>>> mi_switch() at mi_switch+0x275
> >>>>>>> critical_exit() at critical_exit+0x9b
> >>>>>>> spinlock_exit() at spinlock_exit+0x17
> >>>>>>> [several pages of the previous three lines skipped]
> >>>>>>> mi_switch() at mi_switch+0x275
> >>>>>>> critical_exit() at critical_exit+0x9b
> >>>>>>> spinlock_exit() at spinlock_exit+0x17
> >>>>>>> intr_even_schedule_thread() at intr_event_schedule_thread+0xbb
> >>>>>>> ahci_end_transaction() at ahci_end_transaction+0x398
> >>>>>>> ahci_ch_intr() at ahci_ch_intr+0x2b5
> >>>>>>> ahcipoll() at ahcipoll+0x15
> >>>>>>> xpt_polled_action() at xpt_polled_action+0xf7
> >>>>>>>
> >>>>>>> In fact I once discussed with jhb this recursion triggered from a different
> >>>>>>> place.  To quote myself:
> >>>>>>> <avg>    spinlock_exit ->  critical_exit ->  mi_switch ->  kdb_switch ->
> >>>>>>> thread_unlock ->  spinlock_exit ->  critical_exit ->  mi_switch ->  ...
> >>>>>>> <avg>    in the kdb context
> >>>>>>> <avg>    this issue seems to be triggered by td_owepreempt being true at 
> >>>>>>> the time
> >>>>>>> kdb is entered
> >>>>>>> <avg>    and there of course has to be an initial spinlock_exit call 
> >>>>>>> somewhere
> >>>>>>> <avg>    in my case it's because of usb keyboard
> >>>>>>> <avg>    I wonder if it would make sense to clear td_owepreempt right 
> >>>>>>> before
> >>>>>>> calling kdb_switch in mi_switch
> >>>>>>> <avg>    instead of in sched_switch()
> >>>>>>> <avg>    clearing td_owepreempt seems like a scheduler-independent 
> >>>>>>> operation to me
> >>>>>>> <avg>    or is it better to just skip locking in usb when kdb_active is set
> >>>>>>> <avg>    ?
> >>>>>>>
> >>>>>>> The workaround described above should work in this case.
> >>>>>>> Another possibility is to pessimize mtx_unlock_spin() implementations to 
> >>>>>>> check
> >>>>>>> SCHEDULER_STOPPED() and to bypass any further actions in that case.  But 
> >>>>>>> that
> >>>>>>> would add unnecessary overhead to the sunny day code paths.
> >>>>>>>
> >>>>>>> Going further up the stack one can come up with the following proposals:
> >>>>>>> - check SCHEDULER_STOPPED() swi_sched() and return early
> >>>>>>> - do not call swi_sched() from xpt_done() if we somehow know that we are 
> >>>>>>> in a
> >>>>>>> polling mode
> >>>>>>
> >>>>>> There is no flag in CAM now to indicate polling mode, but if needed, it 
> >>>>>> should not be difficult to add one and not call swi_sched().
> >>>>>
> >>>>> I have the following change for eons on my test boxes. Without it,
> >>>>> I simply cannot get _any_ dump.

> >>>> That should be OK for kernel dumping. I was thinking about CAM abusing
> >>>> polling not only for dumping. But looking on cases where it does it now,
> >>>> may be it is better to rewrite them instead.
> >>>
> >>> So, should I interpret your response as 'Reviewed by" ?
> >>
> >> It feels somehow dirty to me. I don't like these global variables. If
> >> you consider it is fine, proceed, I see no much harm. But if not, I can
> >> add polling flag to the CAM. Flip a coin for me. :)
> > You promised to add the polling at summer' meeting in Kiev. Will you do
> > it now ?
> 
> Sorry, I've probably forgot. The patch is attached.

After rebasing on r227637 dumping core from the debugger works
and the backtrace is at least partly usable. PR updated.

Thanks a lot.

Fabian