Date: Thu, 17 Nov 2011 14:40:45 +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: <4EC500CD.6040305@FreeBSD.org> In-Reply-To: <20111117111405.GT50300@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> <4EC4C89A.2040208@FreeBSD.org> <20111117090653.GR50300@deviant.kiev.zoral.com.ua> <4EC4D3DE.8080103@FreeBSD.org> <20111117111405.GT50300@deviant.kiev.zoral.com.ua>
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This is a multi-part message in MIME format. --------------090104080008010605000800 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit 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@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. >>> >>> 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. -- Alexander Motin --------------090104080008010605000800 Content-Type: text/plain; name="cam_poll.patch" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename="cam_poll.patch" Index: cam_sim.h =================================================================== --- cam_sim.h (revision 227580) +++ cam_sim.h (working copy) @@ -104,7 +104,8 @@ u_int32_t flags; #define CAM_SIM_REL_TIMEOUT_PENDING 0x01 #define CAM_SIM_MPSAFE 0x02 -#define CAM_SIM_ON_DONEQ 0x04 +#define CAM_SIM_ON_DONEQ 0x04 +#define CAM_SIM_POLLED 0x08 struct callout callout; struct cam_devq *devq; /* Device Queue to use for this SIM */ int refcount; /* References to the SIM. */ Index: cam_xpt.c =================================================================== --- cam_xpt.c (revision 227580) +++ cam_xpt.c (working copy) @@ -2957,6 +2957,9 @@ mtx_assert(sim->mtx, MA_OWNED); + /* Don't use ISR for this SIM while polling. */ + sim->flags |= CAM_SIM_POLLED; + /* * Steal an opening so that no other queued requests * can get it before us while we simulate interrupts. @@ -2996,6 +2999,9 @@ } else { start_ccb->ccb_h.status = CAM_RESRC_UNAVAIL; } + + /* Use CAM ISR for this SIM again. */ + sim->flags &= ~CAM_SIM_POLLED; } /* @@ -4224,7 +4230,7 @@ TAILQ_INSERT_TAIL(&sim->sim_doneq, &done_ccb->ccb_h, sim_links.tqe); done_ccb->ccb_h.pinfo.index = CAM_DONEQ_INDEX; - if ((sim->flags & CAM_SIM_ON_DONEQ) == 0) { + if ((sim->flags & (CAM_SIM_ON_DONEQ | CAM_SIM_POLLED)) == 0) { mtx_lock(&cam_simq_lock); first = TAILQ_EMPTY(&cam_simq); TAILQ_INSERT_TAIL(&cam_simq, sim, links); --------------090104080008010605000800--
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