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SubjectRe: CPUs blocking in update_blocked_averages -- 4.19.12
Hi Sargun,

On Thu, 27 Dec 2018 at 10:36, Sargun Dhillon <sargun@sargun.me> wrote:
>
> >
> > We're seeing an issue introduced in 4.18, and that still persists in
> > 4.19.12 (we upgraded from 4.9), where update_blocked_averages is
> > blocking. We're running a workload where a majority of our workloads
> > are confined by cgroups. This exhibits itself due to a softlockup on
> > a CPU which is waiting on a lock that's held by
> > update_blocked_averages. I don't have access to any bare metal
> > hardware at the moment, and these are all systems running under KVM.
> > This kernel has CONFIG_FAIR_GROUP_SCHED on.

Your report looks similar to Xie ones:
https://lkml.org/lkml/2018/12/26/541

> >
> > The Softlocked CPU:
> > crash> bt
> > PID: 29314 TASK: ffff99b5f0671f00 CPU: 0 COMMAND: "ag"
> > #0 [ffff99b67b203d58] machine_kexec at ffffffff91a602f3
> > #1 [ffff99b67b203db0] __crash_kexec at ffffffff91b49011
> > #2 [ffff99b67b203e70] panic at ffffffff91a896d1
> > #3 [ffff99b67b203ef8] watchdog_timer_fn at ffffffff91b816af
> > #4 [ffff99b67b203f28] __hrtimer_run_queues at ffffffff91b26359
> > #5 [ffff99b67b203f88] hrtimer_interrupt at ffffffff91b26de2
> > #6 [ffff99b67b203fd8] smp_apic_timer_interrupt at ffffffff92402b06
> > #7 [ffff99b67b203ff0] apic_timer_interrupt at ffffffff92401a9f
> > --- <IRQ stack> ---
> > #8 [ffffa49fa1acbc08] apic_timer_interrupt at ffffffff92401a9f
> > [exception RIP: smp_call_function_many+543]
> > RIP: ffffffff91b3e84f RSP: ffffa49fa1acbcb0 RFLAGS: 00000202
> > RAX: 0000000000000010 RBX: ffff99b67b224fc0 RCX: ffff99b67b6286a0
> > RDX: 0000000000000001 RSI: 0000000000000200 RDI: ffff99b67b224fc8
> > RBP: ffff99b67b224fc8 R8: 0000000000000001 R9: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000025080
> > R13: ffffa49fa1acbd00 R14: 0000000000000001 R15: 0000000000000200
> > ORIG_RAX: ffffffffffffff13 CS: 0010 SS: 0018
> > #9 [ffffa49fa1acbcf8] flush_tlb_mm_range at ffffffff91a7853c
> > #10 [ffffa49fa1acbd90] free_pgtables at ffffffff91c4ac3b
> > #11 [ffffa49fa1acbdc8] unmap_region at ffffffff91c54439
> > #12 [ffffa49fa1acbe90] do_munmap at ffffffff91c56355
> > #13 [ffffa49fa1acbee0] vm_munmap at ffffffff91c56587
> > #14 [ffffa49fa1acbf20] __x64_sys_munmap at ffffffff91c565e2
> > #15 [ffffa49fa1acbf38] do_syscall_64 at ffffffff91a04590
> > #16 [ffffa49fa1acbf50] entry_SYSCALL_64_after_hwframe at ffffffff9240008d
> > RIP: 00007f43e94de747 RSP: 00007f43e91d7e28 RFLAGS: 00000213
> > RAX: ffffffffffffffda RBX: 00000000011a6f30 RCX: 00007f43e94de747
> > RDX: 0000000000000001 RSI: 0000000000000ade RDI: 00007f43ea47d000
> > RBP: 0000000000000005 R8: 00007f43e00008c0 R9: 00007f43e91d7d00
> > R10: 00000000011a53b0 R11: 0000000000000213 R12: 00007f43ea47d000
> > R13: 0000000000000ade R14: 00007f43e91d89c0 R15: 0000000000000060
> > ORIG_RAX: 000000000000000b CS: 0033 SS: 002b
> >
> > This is waiting on CPU 16:
> > crash> bt
> > PID: 29299 TASK: ffff99b5dffd1f00 CPU: 16 COMMAND: "ag"
> > #0 [fffffe00002c8e38] crash_nmi_callback at ffffffff91a54202
> > #1 [fffffe00002c8e50] nmi_handle at ffffffff91a32ee2
> > #2 [fffffe00002c8ea0] default_do_nmi at ffffffff91a333ea
> > #3 [fffffe00002c8ec0] do_nmi at ffffffff91a33553
> > #4 [fffffe00002c8ef0] end_repeat_nmi at ffffffff924014e8
> > [exception RIP: native_queued_spin_lock_slowpath+33]
> > RIP: ffffffff91af2dc1 RSP: ffffa49fa1a93948 RFLAGS: 00000002
> > RAX: 0000000000000001 RBX: ffff99b67b763f40 RCX: 000000004f19a57a
> > RDX: 0000000000000001 RSI: 0000000000000001 RDI: ffff99b67b763f40
> > RBP: 0000000000000082 R8: 0000000000000001 R9: 0000000000000000
> > R10: ffffa49fa1a938f8 R11: ffffffff91aca4a7 R12: 0000000000000015
> > R13: ffff99b67b764940 R14: ffffa49fa1a93a60 R15: 0000000000000015
> > ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0000
> > --- <NMI exception stack> ---
> > #5 [ffffa49fa1a93948] native_queued_spin_lock_slowpath at ffffffff91af2dc1
> > #6 [ffffa49fa1a93948] do_raw_spin_lock at ffffffff91af48dd
> > #7 [ffffa49fa1a93958] _raw_spin_lock_irqsave at ffffffff923c1154
> > #8 [ffffa49fa1a93978] update_blocked_averages at ffffffff91aca4a7
> > #9 [ffffa49fa1a93a10] update_nohz_stats at ffffffff91acaa3d
> > #10 [ffffa49fa1a93a20] find_busiest_group at ffffffff91ad2137
> > #11 [ffffa49fa1a93bb8] load_balance at ffffffff91ad2be0
> > #12 [ffffa49fa1a93cf0] pick_next_task_fair at ffffffff91ad41e6
> > #13 [ffffa49fa1a93da0] mark_held_locks at ffffffff91aeebe4
> > #14 [ffffa49fa1a93de8] schedule at ffffffff923b9ec2
> > #15 [ffffa49fa1a93df8] rwsem_down_read_failed at ffffffff923be859
> > #16 [ffffa49fa1a93e80] call_rwsem_down_read_failed at ffffffff923ae944
> > #17 [ffffa49fa1a93ec8] down_read at ffffffff923bdf0b
> > #18 [ffffa49fa1a93ee0] __do_page_fault at ffffffff91a71083
> > #19 [ffffa49fa1a93f50] async_page_fault at ffffffff924010ee
> > RIP: 00007f43e945f786 RSP: 00007ffde2b50070 RFLAGS: 00010206
> > RAX: 0000000000015f41 RBX: 00007f43e97a1b20 RCX: 0000000000000111
> > RDX: 00000000011affb0 RSI: 00000000011b00c0 RDI: 0000000000000000
> > RBP: 0000000000000111 R8: 00007f43e97a1bd8 R9: 0000000000000000
> > R10: 00007f43e97a1b78 R11: 00007f43e97a1b78 R12: 00007f43e97a1b78
> > R13: 00007f43e97a1b78 R14: 0000000000002710 R15: 00007f43e97a1c78
> > ORIG_RAX: ffffffffffffffff CS: 0033 SS: 002b
> >
> > Which is waiting on a lock held by CPU owned by CPU 9:
> > crash> struct raw_spinlock_t ffff99b67b763f40
> > struct raw_spinlock_t {
> > raw_lock = {
> > {
> > val = {
> > counter = 1
> > },
> > {
> > locked = 1 '\001',
> > pending = 0 '\000'
> > },
> > {
> > locked_pending = 1,
> > tail = 0
> > }
> > }
> > },
> > magic = 3735899821,
> > owner_cpu = 9,
> > owner = 0xffff99b66e7a8000,
> > dep_map = {
> > key = 0xffffffff9331d120,
> > class_cache = {0xffffffff93a13da0, 0x0},
> > name = 0xffffffff92a7d9ad "&rq->lock"
> > }
> > }
> >
> > Which is hanging out in update_blocked_averages:
> > crash> bt
> > PID: 0 TASK: ffff99b66e7a8000 CPU: 9 COMMAND: "swapper/9"
> > #0 [fffffe0000194e38] crash_nmi_callback at ffffffff91a54202
> > #1 [fffffe0000194e50] nmi_handle at ffffffff91a32ee2
> > #2 [fffffe0000194ea0] default_do_nmi at ffffffff91a33362
> > #3 [fffffe0000194ec0] do_nmi at ffffffff91a33553
> > #4 [fffffe0000194ef0] end_repeat_nmi at ffffffff924014e8
> > [exception RIP: lock_is_held_type+70]
> > RIP: ffffffff91aecec6 RSP: ffff99b67b443e20 RFLAGS: 00000046
> > RAX: 0000000000000001 RBX: ffff99b66e7a8000 RCX: 00000000000000a8
> > RDX: 0000000000000001 RSI: 00000000ffffffff RDI: 0000000000000046
> > RBP: 0000000000000046 R8: 0000000000000004 R9: 0000000000000000
> > R10: ffff99b67b443e38 R11: ffffffff91aca4a7 R12: ffff99b67b763f40
> > R13: ffff99b67b764940 R14: 0000000000000000 R15: ffff99b6193d9200
> > ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
> > --- <NMI exception stack> ---
> > #5 [ffff99b67b443e20] lock_is_held_type at ffffffff91aecec6
> > #6 [ffff99b67b443e30] update_blocked_averages at ffffffff91aca841
> > #7 [ffff99b67b443ec8] update_nohz_stats at ffffffff91acaa3d
> > #8 [ffff99b67b443ed8] _nohz_idle_balance at ffffffff91ad39f7
> > #9 [ffff99b67b443f70] __softirqentry_text_start at ffffffff926000d1
> > #10 [ffff99b67b443fd0] irq_exit at ffffffff91a911f5
> > #11 [ffff99b67b443fd8] smp_apic_timer_interrupt at ffffffff92402b2c
> > #12 [ffff99b67b443ff0] apic_timer_interrupt at ffffffff92401a9f
> > --- <IRQ stack> ---
> > #13 [ffffa49f98993df8] apic_timer_interrupt at ffffffff92401a9f
> > [exception RIP: mwait_idle+108]
> > RIP: ffffffff923c02fc RSP: ffffa49f98993ea8 RFLAGS: 00000246
> > RAX: 0000000000000000 RBX: 0000000000000009 RCX: 0000000000000000
> > RDX: 0000000000000001 RSI: 0000000000000001 RDI: ffff99b66e7a8000
> > RBP: 0000000000000009 R8: 0000000000000001 R9: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000000 R12: ffff99b66e7a8000
> > R13: 0000000000000000 R14: 0000000000000000 R15: ffff99b66e7a8000
> > ORIG_RAX: ffffffffffffff13 CS: 0010 SS: 0018
> > #14 [ffffa49f98993ec0] do_idle at ffffffff91ac6ba3
> > #15 [ffffa49f98993f10] cpu_startup_entry at ffffffff91ac6f4f
> > #16 [ffffa49f98993f30] start_secondary at ffffffff91a55759
> > #17 [ffffa49f98993f50] secondary_startup_64 at ffffffff91a000d4
> >
> > ---
> >
> > Lockdep tells me nothing, which makes me think, somehow is the code in
> > for_each_leaf_cfs_rq_safe looping forever?
>
> On further investigation from another crash dump, it looks like (where
> 0xffff9680bb4e3f40 = cpu_rq(11)) --
> crash> struct rq.leaf_cfs_rq_list 0xffff9680bb4e3f40
> leaf_cfs_rq_list = {
> next = 0xffff9680622ed980,
> prev = 0xffff9680bb4e4180
> }
> crash> struct list_head 0xffff9680622ed980
> struct list_head {
> next = 0xffff9680622ed980,
> prev = 0xffff9680622ed980
> }
>
> ---
> I instrumented update_blocked_averages with the start time of the
> function, to keep track of it with the scheduler clock, and it
> coincides at almost exactly 5.5 seconds, way longer than it should be.
>
>
> It looks like it never terminates because it stays in this loop
> forever. I'm not sure how things can get into this state, or even if
> this was the state to start with, and this isn't the result of some
> kind of concurrency skew or other shenanigans.

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