Messages in this thread | | | Date | Wed, 26 Jan 2022 08:12:35 -0600 | From | Corey Minyard <> | Subject | Re: Possible reproduction of CSD locking issue |
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On Wed, Jan 26, 2022 at 07:56:43AM -0600, Corey Minyard wrote: > On Wed, Jan 26, 2022 at 07:08:22AM +0100, Juergen Gross wrote: > > On 25.01.22 19:27, Corey Minyard wrote: > > > We have a customer that had been seeing CSD lock issues on a Centos 7 > > > kernel (unfortunately). I couldn't find anything or any kernel changes > > > that might fix it, so I was consdering it was the CSD locking issue you > > > have been chasing for a while. > > > > Is this on bare metal or in a virtualized environment? > > This is bare metal. > > I do think I know what happened. Here's my analysis...
Well, never mind, I missed that preemption is disabled in this code. The lack of pinged entries is still a little confusing.
-corey
> > csd: Detected non-responsive CSD lock (#1) on CPU#3, waiting 5000000042 ns for CPU#55 flush_tlb_func+0x0/0xb0(0xffff8e0b3e2afbe8). > csd: CSD lock (#1) unresponsive. > csd: cnt(0000000): 0000->0000 queue > csd: cnt(0000001): ffff->0037 idle > > The above means that these events weren't seen, I think. We can > ignore them in any case. > > csd: cnt(63d8dd8): 0003->0037 ipi > csd: cnt(63d8dd9): 0003->0037 ping > csd: cnt(63d8dda): 0003->ffff pinged > > This is a little confusing. The first two lines have to be from > __smp_call_single_queue_debug. The last line has to be from > smp_call_function_many. But you never see the pinged from > __smp_call_single_queue_debug. > > csd: cnt(63d8dea): 0035->0037 pinged > > The tail of CPU 53 sending an IPI to CPU 55 in > __smp_call_single_queue_debug. > > csd: cnt(63d8deb): ffff->0037 gotipi > csd: cnt(63d8dec): ffff->0037 handle > csd: cnt(63d8ded): ffff->0037 dequeue (src CPU 0 == empty) > csd: cnt(63d8dee): ffff->0037 hdlend (src CPU 0 == early) > > CPU 55 is handling the IPI(s) it was sent earlier. > > csd: cnt(63d8e1f): 0003->0037 queue > csd: cnt(63d8e20): 0003->0037 ipi > csd: cnt(63d8e21): 0003->0037 ping > > In __smp_call_single_queue_debug CPU 3 sends another message to > CPU 55 and sends an IPI. But there should be a pinged entry > after this. > > csd: cnt(63d8e22): 0003->0037 queue > csd: cnt(63d8e23): 0003->0037 noipi > > In __smp_call_single_queue_debug CPU 3 sends another message to > CPU 55, but no IPI is required because one is already pending. > > I deleted the timestamps and the rest of the log, but a second > IPI is sent and CPU 55 wakes up immediately. That tells me that > CPU 55 wasn't stuck for some reason (interrupt storm, long time > with interrupts disabled). Though maybe not, with your insight > below? > > Looking at the code, it appears this code runs with interrupts and > preemption enabled, so they can be switched out at any time here. I can > only see one possibility from the above sequence: > > * CPU 3 queues the list item but is switched out before sending > the IPI (no pinged in the sequence). > > * Another thread on CPU 3 is switched in and finishes a send > many. > > * CPU 53 sends an IPI to CPU 55 and wakes it up. > > * CPU 55 handles the IPI. > > * Another thread on CPU 3 then sends another message to CPU 55, > enqueues the link, but is switched out before actually sending > the IPI (again, no pinged in the sequence). > > * Yet another thread on CPU 3 sends a message, but the item > is already queued and thus it does not send an IPI. > > * Something on CPU 3 prevents the first two threads from sending > the IPI. > > So assuming it's not a hardware bug, it appears the problem is > process starvation on CPU 3. Knowing our customer's > architecture, this is actually a possibility. This could be > pretty easily solved by turning off interrupts (or preemption?) around > the code that enqueues the link and sends the IPI. > > > > > The bug I was hunting occurred when running under Xen, and in the end I > > found a scheduling problem in the Xen hypervisor. > > > > > > > > So I backported the debug patches. And of course, they stopped seeing > > > the issue, at least as much, and they had trouble with the extra CPU > > > time the debug code took. But they just reproduced it. Here are the > > > logs: > > > > > > Jan 23 23:39:43 worker0 kernel: [285737.522743] csd: Detected non-responsive CSD lock (#1) on CPU#3, waiting 5000000042 ns for CPU#55 flush_tlb_func+0x0/0xb0(0xffff8e0b3e2afbe8). > > > Jan 23 23:39:43 worker0 kernel: [285737.522744] csd: CSD lock (#1) unresponsive. > > > Jan 23 23:39:43 worker0 kernel: [285737.522747] csd: cnt(0000000): 0000->0000 queue > > > Jan 23 23:39:43 worker0 kernel: [285737.522748] csd: cnt(0000001): ffff->0037 idle > > > Jan 23 23:39:43 worker0 kernel: [285737.522749] csd: cnt(63d8dd8): 0003->0037 ipi > > > Jan 23 23:39:43 worker0 kernel: [285737.522750] csd: cnt(63d8dd9): 0003->0037 ping > > > Jan 23 23:39:43 worker0 kernel: [285737.522750] csd: cnt(63d8dda): 0003->ffff pinged > > > Jan 23 23:39:43 worker0 kernel: [285737.522751] csd: cnt(63d8dea): 0035->0037 pinged > > > Jan 23 23:39:43 worker0 kernel: [285737.522752] csd: cnt(63d8deb): ffff->0037 gotipi > > > Jan 23 23:39:43 worker0 kernel: [285737.522752] csd: cnt(63d8dec): ffff->0037 handle > > > Jan 23 23:39:43 worker0 kernel: [285737.522753] csd: cnt(63d8ded): ffff->0037 dequeue (src CPU 0 == empty) > > > Jan 23 23:39:43 worker0 kernel: [285737.522754] csd: cnt(63d8dee): ffff->0037 hdlend (src CPU 0 == early) > > > Jan 23 23:39:43 worker0 kernel: [285737.522754] csd: cnt(63d8e1f): 0003->0037 queue > > > Jan 23 23:39:43 worker0 kernel: [285737.522755] csd: cnt(63d8e20): 0003->0037 ipi > > > Jan 23 23:39:43 worker0 kernel: [285737.522756] csd: cnt(63d8e21): 0003->0037 ping > > > Jan 23 23:39:43 worker0 kernel: [285737.522756] csd: cnt(63d8e22): 0003->0037 queue > > > Jan 23 23:39:43 worker0 kernel: [285737.522757] csd: cnt(63d8e23): 0003->0037 noipi > > > Jan 23 23:39:43 worker0 kernel: [285737.522757] csd: cnt now: 63fe4cd > > > Jan 23 23:39:43 worker0 kernel: [285737.522758] Task dump for CPU 55: > > > Jan 23 23:39:43 worker0 kernel: [285737.522761] kubelet R running task 0 277695 1 0x00080000 > > > Jan 23 23:39:43 worker0 kernel: [285737.522761] Call Trace: > > > Jan 23 23:39:43 worker0 kernel: [285737.522769] [<ffffffff84376b6a>] ? __schedule+0x46a/0x990 > > > Jan 23 23:39:43 worker0 kernel: [285737.522774] [<ffffffff83db6353>] ? context_tracking_user_enter+0x13/0x20 > > > Jan 23 23:39:43 worker0 kernel: [285737.522776] [<ffffffff843775b5>] ? schedule_user+0x45/0x50 > > > Jan 23 23:39:43 worker0 kernel: [285737.522779] [<ffffffff8437b518>] ? retint_careful+0x16/0x34 > > > Jan 23 23:39:43 worker0 kernel: [285737.522780] csd: Re-sending CSD lock (#1) IPI from CPU#03 to CPU#55 > > > Jan 23 23:39:43 worker0 kernel: [285737.522788] CPU: 3 PID: 54671 Comm: runc:[2:INIT] Kdump: loaded Tainted: G OE ------------ T 3.10.0-1062.12.1.rt56.1042.mvista.test.14.el7.x86_64 #1 > > > Jan 23 23:39:43 worker0 kernel: [285737.522789] Hardware name: Dell Inc. PowerEdge R740/0YWR7D, BIOS 2.9.4 11/06/2020 > > > Jan 23 23:39:43 worker0 kernel: [285737.522789] Call Trace: > > > Jan 23 23:39:43 worker0 kernel: [285737.522793] [<ffffffff843718ba>] dump_stack+0x19/0x1b > > > Jan 23 23:39:43 worker0 kernel: [285737.522798] [<ffffffff83d0bcd8>] __csd_lock_wait+0x1a8/0x2a0 > > > Jan 23 23:39:43 worker0 kernel: [285737.522800] [<ffffffff83c6d870>] ? leave_mm+0x120/0x120 > > > Jan 23 23:39:43 worker0 kernel: [285737.522802] [<ffffffff83d0bfa4>] smp_call_function_single+0xc4/0x1b0 > > > Jan 23 23:39:43 worker0 kernel: [285737.522804] [<ffffffff83c6d870>] ? leave_mm+0x120/0x120 > > > Jan 23 23:39:43 worker0 kernel: [285737.522809] [<ffffffff83e2684b>] ? page_counter_uncharge+0x3b/0x70 > > > Jan 23 23:39:43 worker0 kernel: [285737.522811] [<ffffffff83d0c614>] smp_call_function_many+0x344/0x380 > > > Jan 23 23:39:43 worker0 kernel: [285737.522813] [<ffffffff83c6d870>] ? leave_mm+0x120/0x120 > > > Jan 23 23:39:43 worker0 kernel: [285737.522816] [<ffffffff83c6da38>] native_flush_tlb_others+0xb8/0xc0 > > > Jan 23 23:39:43 worker0 kernel: [285737.522818] [<ffffffff83c6dc25>] flush_tlb_page+0x65/0xf0 > > > Jan 23 23:39:43 worker0 kernel: [285737.522821] [<ffffffff83dfdf98>] ptep_clear_flush+0x68/0xa0 > > > Jan 23 23:39:43 worker0 kernel: [285737.522825] [<ffffffff83de6806>] wp_page_copy.isra.83+0x3d6/0x650 > > > Jan 23 23:39:43 worker0 kernel: [285737.522828] [<ffffffff83de8cb4>] do_wp_page+0xb4/0x710 > > > Jan 23 23:39:43 worker0 kernel: [285737.522832] [<ffffffff83decbb4>] handle_mm_fault+0x884/0x1340 > > > Jan 23 23:39:43 worker0 kernel: [285737.522835] [<ffffffff83cd7799>] ? update_cfs_shares+0xa9/0xf0 > > > Jan 23 23:39:43 worker0 kernel: [285737.522839] [<ffffffff8437efc3>] __do_page_fault+0x213/0x5a0 > > > Jan 23 23:39:43 worker0 kernel: [285737.522841] [<ffffffff8437f385>] do_page_fault+0x35/0x90 > > > Jan 23 23:39:43 worker0 kernel: [285737.522842] [<ffffffff8437b728>] page_fault+0x28/0x30 > > > Jan 23 23:39:43 worker0 kernel: [285737.522845] csd: CSD lock (#1) got unstuck on CPU#03, CPU#55 released the lock. > > > > > > Hopefully this is the issue you are chasing and not something else. > > > I've been studying them to see what they mean, but I thought you might > > > be interested to get them asap. > > > > As said before: I've solved my problem. > > > > But I can share some more insight from my experience: > > > > Don't be so sure that resending the IPI woke up the cpu again. It might > > have been the NMI for getting the target cpu's stacktrace which resulted > > in letting it run again. I missed this possibility in the beginning and > > this was the reason I dismissed the Xen scheduling issue too early. > > Hmm. That's non-obvious, obviously. I'm not sure how sending an NMI > would unstick something like this, but I'll take your word for it. > I think my above analysis is correct based on the sequence, but this > gives me something else to consider. > > Thanks for the input. > > -corey > > > > > > > Juergen > > > > >
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