On 01/14/2016 01:19 PM, Christian Borntraeger wrote:
> Folks,
>
> With 4.4 I can easily bring the system into a hang like situation by
> putting stress on the cgroup_threadgroup rwsem. (e.g. starting/stopping
> kvm guests via libvirt and many vCPUs). Here is my preliminary analysis:
>
> When the hang happens, the system is idle for all CPUs. There are some
> processes waiting for the cgroup_thread_rwsem, e.g.
>
> crash> bt 87399
> PID: 87399 TASK: faef084998 CPU: 59 COMMAND: "systemd-udevd"
> #0 [f9e762fc88] __schedule at 83b2cc
> #1 [f9e762fcf0] schedule at 83ba26
> #2 [f9e762fd08] rwsem_down_read_failed at 83fb64
> #3 [f9e762fd68] percpu_down_read at 1bdf56
> #4 [f9e762fdd0] exit_signals at 1742ae
> #5 [f9e762fe00] do_exit at 163be0
> #6 [f9e762fe60] do_group_exit at 165c62
> #7 [f9e762fe90] __wake_up_parent at 165d00
> #8 [f9e762fea8] system_call at 842386
>
> of course, any new process would wait for the same lock during fork.
>
> Looking at the rwsem, while all CPUs are idle, it appears that the lock
> is taken for write:
>
> crash> print /x cgroup_threadgroup_rwsem.rw_sem
> $8 = {
> count = 0xfffffffe00000001,
> [..]
> owner = 0xfabf28c998,
> }
>
> Looking at the owner field:
>
> crash> bt 0xfabf28c998
> PID: 11867 TASK: fabf28c998 CPU: 42 COMMAND: "libvirtd"
> #0 [fadeccb5e8] __schedule at 83b2cc
> #1 [fadeccb650] schedule at 83ba26
> #2 [fadeccb668] schedule_timeout at 8403c6
> #3 [fadeccb748] wait_for_common at 83c850
> #4 [fadeccb7b8] flush_work at 18064a
> #5 [fadeccb8d8] lru_add_drain_all at 2abd10
> #6 [fadeccb938] migrate_prep at 309ed2
> #7 [fadeccb950] do_migrate_pages at 2f7644
> #8 [fadeccb9f0] cpuset_migrate_mm at 220848
> #9 [fadeccba58] cpuset_attach at 223248
> #10 [fadeccbaa0] cgroup_taskset_migrate at 21a678
> #11 [fadeccbaf8] cgroup_migrate at 21a942
> #12 [fadeccbba0] cgroup_attach_task at 21ab8a
> #13 [fadeccbc18] __cgroup_procs_write at 21affa
> #14 [fadeccbc98] cgroup_file_write at 216be0
> #15 [fadeccbd08] kernfs_fop_write at 3aa088
> #16 [fadeccbd50] __vfs_write at 319782
> #17 [fadeccbe08] vfs_write at 31a1ac
> #18 [fadeccbe68] sys_write at 31af06
> #19 [fadeccbea8] system_call at 842386
> PSW: 0705100180000000 000003ff9438f9f0 (user space)
>
> it appears that the write holder scheduled away and waits
> for a completion. Now what happens is, that the write lock
> holder finally calls flush_work for the lru_add_drain_all
> work.
So what's happening is that libvirtd wants to move some processes in the
cgroup subtree and it to the respective cgroup file. So
cgroup_threadgroup_rwsem is acquired in __cgroup_procs_write, then as
part of this process the pages for that process have to be migrated,
hence the do_migrate_pages. And this call chain boils down to calling
lru_add_drain_cpu on every cpu.
>
> As far as I can see, this work is now tries to create a new kthread
> and waits for that, as the backtrace for the kworker on that cpu has:
>
> PID: 81913 TASK: fab5356220 CPU: 42 COMMAND: "kworker/42:2"
> #0 [fadd6d7998] __schedule at 83b2cc
> #1 [fadd6d7a00] schedule at 83ba26
> #2 [fadd6d7a18] schedule_timeout at 8403c6
> #3 [fadd6d7af8] wait_for_common at 83c850
> #4 [fadd6d7b68] wait_for_completion_killable at 83c996
> #5 [fadd6d7b88] kthread_create_on_node at 1876a4
> #6 [fadd6d7cc0] create_worker at 17d7fa
> #7 [fadd6d7d30] worker_thread at 17fff0
> #8 [fadd6d7da0] kthread at 187884
> #9 [fadd6d7ea8] kernel_thread_starter at 842552
>
> Problem is that kthreadd then needs the cgroup lock for reading,
> while libvirtd still has the lock for writing.
>
> crash> bt 0xfaf031e220
> PID: 2 TASK: faf031e220 CPU: 40 COMMAND: "kthreadd"
> #0 [faf034bad8] __schedule at 83b2cc
> #1 [faf034bb40] schedule at 83ba26
> #2 [faf034bb58] rwsem_down_read_failed at 83fb64
> #3 [faf034bbb8] percpu_down_read at 1bdf56
> #4 [faf034bc20] copy_process at 15eab6
> #5 [faf034bd08] _do_fork at 160430
> #6 [faf034bdd0] kernel_thread at 160a82
> #7 [faf034be30] kthreadd at 188580
> #8 [faf034bea8] kernel_thread_starter at 842552
>
> BANG.kthreadd waits for the lock that libvirtd hold, and libvirtd waits
> for kthreadd to finish some task
I don't see percpu_down_read being invoked from copy_process. According
to LXR, this semaphore is used only in __cgroup_procs_write and
cgroup_update_dfl_csses. And cgroup_update_dfl_csses is invoked when
cgroup.subtree_control is written to. And I don't see this happening in
this call chain.
Going from there I'm questioning whether the failure to fork the from
kthreadd is indeed related to the cgroup semaphore. Can you try and
inspect the stacks for process 0xfaf031e220 to see if the address of the
cgroup rwsemaphore can be found there?
>
> Reverting 001dac627ff374 ("locking/percpu-rwsem: Make use of the rcu_sync
> infrastructure") does not help, so it does not seem to be related to the
> rcu_sync rework.
>
> Any ideas, questions (dump is still available)
>
> PS: not sure if lockdep could detect such a situation. it is running but silent.
>
>
> Christian
>
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