** Also affects: linux-azure-4.15 (Ubuntu)
Importance: Undecided
Status: New
** Changed in: linux-azure-4.15 (Ubuntu)
Status: New => Fix Committed
** Changed in: linux-azure-4.15 (Ubuntu Bionic)
Status: New => Fix Committed
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https://bugs.launchpad.net/bugs/1882623
Title:
VM enter into hung status after triggering a crash
Status in linux-azure package in Ubuntu:
In Progress
Status in linux-azure-4.15 package in Ubuntu:
Fix Committed
Status in linux-azure source package in Bionic:
Fix Committed
Status in linux-azure-4.15 source package in Bionic:
Fix Committed
Bug description:
[Impact]
* When kdumping on trusty/4.15 in an Azure instance, we observe quite
frequently a stall on the kdump kernel, it gets blocked and soon we
see a stack like the following:
[ 65.452007] INFO: rcu_sched detected stalls on CPUs/tasks:
[ 65.456004] 1-...!: (0 ticks this GP) idle=488/0/0 softirq=1/1 fqs=0
[ 65.456004] (detected by 0, t=15002 jiffies, g=707, c=706, q=8457)
[ 65.456004] rcu_sched kthread starved for 15002 jiffies! g707 c706 f0x0
RCU_GP_WAIT_FQS(3) ->state=0x402 ->cpu=1
* By using the Azure serial console, we collected a sysrq-w when the issue
happens:
[ 529.515013] sysrq: Show Blocked State
[ 529.517730] task PC stack pid father
[ 529.519006] kworker/u4:2 D 0 94 2 0x80000000
[ 529.519006] Workqueue: events_unbound fsnotify_mark_destroy_workfn
[ 529.519006] Call Trace:
[ 529.519006] __schedule+0x292/0x880
[ 529.519006] schedule+0x36/0x80
[ 529.519006] schedule_timeout+0x1d5/0x2f0
[ 529.519006] ? check_preempt_wakeup+0x162/0x260
[ 529.519006] wait_for_completion+0xa5/0x110
[ 529.519006] ? wake_up_q+0x80/0x80
[ 529.519006] __synchronize_srcu.part.14+0x67/0x80
[ 529.519006] ? trace_raw_output_rcu_utilization+0x50/0x50
[ 529.519006] ? __switch_to_asm+0x41/0x70
[ 529.519006] synchronize_srcu+0xd1/0xd6
[ 529.519006] fsnotify_mark_destroy_workfn+0x6d/0xc0
[ 529.519006] process_one_work+0x14e/0x390
[ 529.519006] worker_thread+0x1cc/0x3d0
[ 529.519006] kthread+0x105/0x140
[ 529.519006] ? max_active_store+0x60/0x60
[ 529.519006] ? kthread_bind+0x20/0x20
[ 529.519006] ret_from_fork+0x35/0x40
[ 529.519006] udevadm D 0 544 1 0x00000000
[ 529.519006] Call Trace:
[ 529.519006] __schedule+0x292/0x880
[ 529.519006] schedule+0x36/0x80
[ 529.519006] schedule_timeout+0x1d5/0x2f0
[ 529.519006] ? try_to_wake_up+0x4a/0x460
[ 529.519006] ? try_to_wake_up+0x4a/0x460
[ 529.519006] wait_for_completion+0xa5/0x110
[ 529.519006] ? wake_up_q+0x80/0x80
[ 529.519006] __flush_work.isra.29+0x119/0x1b0
[ 529.519006] ? destroy_worker+0x90/0x90
[ 529.519006] flush_delayed_work+0x3f/0x50
[ 529.519006] fsnotify_wait_marks_destroyed+0x15/0x20
[ 529.519006] fsnotify_destroy_group+0x4e/0xc0
[ 529.519006] inotify_release+0x1e/0x50
[ 529.519006] __fput+0xea/0x220
[ 529.519006] ____fput+0xe/0x10
[ 529.519006] task_work_run+0x8c/0xb0
[ 529.519006] exit_to_usermode_loop+0x70/0xa9
[ 529.519006] do_syscall_64+0x1b5/0x1e0
[ 529.519006] entry_SYSCALL_64_after_hwframe+0x41/0xa6
[ 529.519006] dhclient D 0 573 572 0x00000000
[ 529.519006] Call Trace:
[ 529.519006] __schedule+0x292/0x880
[ 529.519006] schedule+0x36/0x80
[ 529.519006] schedule_timeout+0x1d5/0x2f0
[ 529.519006] ? aa_profile_af_perm+0xb4/0xf0
[ 529.519006] wait_for_completion+0xa5/0x110
[ 529.519006] ? wake_up_q+0x80/0x80
[ 529.519006] __wait_rcu_gp+0x123/0x150
[ 529.519006] synchronize_sched+0x4e/0x60
[ 529.519006] ? __call_rcu+0x2f0/0x2f0
[ 529.519006] ? trace_raw_output_rcu_utilization+0x50/0x50
[ 529.519006] synchronize_net+0x1c/0x30
[ 529.519006] __unregister_prot_hook+0xcd/0xf0
[ 529.519006] packet_do_bind+0x1bd/0x250
[ 529.519006] packet_bind+0x2f/0x50
[ 529.519006] SYSC_bind+0xd8/0x110
[ 529.519006] ? sock_alloc_file+0x91/0x130
[ 529.519006] SyS_bind+0xe/0x10
[ 529.519006] do_syscall_64+0x80/0x1e0
[ 529.519006] entry_SYSCALL_64_after_hwframe+0x41/0xa6
* Bisecting mainline kernels, we found that v4.17-rc1 didn't reproduce
the issue, whereas v4.16 reproduced. Then, a fine-grained git bisect
led us to the fix - the following patch, when backported to a
problematic version, fixes the issue: d8e462e19305 ("Drivers: hv:
vmbus: Implement Direct Mode for stimer0")
* In Azure/Hyper-V, before the aforementioned commit, timer interrupts
were passed to the hypervisor through a vmbus message, a mechanism of
communication of hyper-v guests/hypervisor. With the patch, a check is
made (through MSR-like mechanism) and if the hypervisor supports, a
direct timer IRQ mechanism is put in-place instead of the vmbus
channel.
* Our theory is that on kdump kernel, specially due to the single cpu
nature, the vmbus-messaged timer IRQ could interfere with scheduling
and create a dead-lock condition, which is what we observe from the
stack traces. Hence, we hereby propose to backport such patch to azure
4.15 kernels.
* Our hypothesis about why this wasn't observed in Bionic is that
changes in the kdump mechanism make it start earlier, and this might
hide the schedule issue. Lack of testing also could be the cause of
the non-observability.
[Test Case]
* Perform a kdump in Trusty/4.15 and after some rounds, you should see
the mentioned RCU stack trace. With the patch, no issue is observed,
and we could perform ~30 kdumps in a row.
[Where problems could occur]
* Since the patch touches IRQ code (although restricted to Hyper-v),
it's a somewhat invasive change. It could lead to issues in the IRQ
subsystem, although the patch is upstream since 2018 and no fixes for
that were found (we count on Fixes tag for that).
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