Re: Stop scheduler on panic

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.
>>>>
>>>> Fabian
>>>
>>> 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.
> 
> diff --git a/sys/cam/cam_xpt.c b/sys/cam/cam_xpt.c
> index 10b89c7..a38e42f 100644
> --- a/sys/cam/cam_xpt.c
> +++ b/sys/cam/cam_xpt.c
> _at__at_ -4230,7 +4230,7 _at__at_ xpt_done(union ccb *done_ccb)
>  			TAILQ_INSERT_TAIL(&cam_simq, sim, links);
>  			mtx_unlock(&cam_simq_lock);
>  			sim->flags |= CAM_SIM_ON_DONEQ;
> -			if (first)
> +			if (first && panicstr == NULL)
>  				swi_sched(cambio_ih, 0);
>  		}
>  	}

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.

-- 
Alexander Motin