Date: Thu, 17 Nov 2011 10:40:58 +0200 From: Alexander Motin <mav@FreeBSD.org> To: Kostik Belousov <kostikbel@gmail.com> Cc: freebsd-current@freebsd.org, Andriy Gapon <avg@freebsd.org> Subject: Re: Stop scheduler on panic Message-ID: <4EC4C89A.2040208@FreeBSD.org> In-Reply-To: <20111117081533.GP50300@deviant.kiev.zoral.com.ua> References: <20111113083215.GV50300@deviant.kiev.zoral.com.ua> <20111116202714.5ee4bd53@fabiankeil.de> <4EC43764.1020202@FreeBSD.org> <4EC4423A.3020904@FreeBSD.org> <20111117081533.GP50300@deviant.kiev.zoral.com.ua>
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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@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 > @@ -4230,7 +4230,7 @@ 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
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