cgroup2 freezer and kvm_vm_worker_thread()

[Tejun Heo <tj@kernel.org>]

Hello,

Luca is reporting that cgroups which have kvm instances inside never
complete freezing. This can be trivially reproduced:

  root@test ~# mkdir /sys/fs/cgroup/test
  root@test ~# echo $fish_pid > /sys/fs/cgroup/test/cgroup.procs
  root@test ~# qemu-system-x86_64 --nographic -enable-kvm

and in another terminal:

  root@test ~# echo 1 > /sys/fs/cgroup/test/cgroup.freeze
  root@test ~# cat /sys/fs/cgroup/test/cgroup.events
  populated 1
  frozen 0
  root@test ~# for i in (cat /sys/fs/cgroup/test/cgroup.threads); echo $i; cat /proc/$i/stack; end 
  2070
  [<0>] do_freezer_trap+0x42/0x70
  [<0>] get_signal+0x4da/0x870
  [<0>] arch_do_signal_or_restart+0x1a/0x1c0
  [<0>] syscall_exit_to_user_mode+0x73/0x120
  [<0>] do_syscall_64+0x87/0x140
  [<0>] entry_SYSCALL_64_after_hwframe+0x76/0x7e
  2159
  [<0>] do_freezer_trap+0x42/0x70
  [<0>] get_signal+0x4da/0x870
  [<0>] arch_do_signal_or_restart+0x1a/0x1c0
  [<0>] syscall_exit_to_user_mode+0x73/0x120
  [<0>] do_syscall_64+0x87/0x140
  [<0>] entry_SYSCALL_64_after_hwframe+0x76/0x7e
  2160
  [<0>] do_freezer_trap+0x42/0x70
  [<0>] get_signal+0x4da/0x870
  [<0>] arch_do_signal_or_restart+0x1a/0x1c0
  [<0>] syscall_exit_to_user_mode+0x73/0x120
  [<0>] do_syscall_64+0x87/0x140
  [<0>] entry_SYSCALL_64_after_hwframe+0x76/0x7e
  2161
  [<0>] kvm_nx_huge_page_recovery_worker+0xea/0x680
  [<0>] kvm_vm_worker_thread+0x8f/0x2b0
  [<0>] kthread+0xe8/0x110
  [<0>] ret_from_fork+0x33/0x40
  [<0>] ret_from_fork_asm+0x1a/0x30
  2164
  [<0>] do_freezer_trap+0x42/0x70
  [<0>] get_signal+0x4da/0x870
  [<0>] arch_do_signal_or_restart+0x1a/0x1c0
  [<0>] syscall_exit_to_user_mode+0x73/0x120
  [<0>] do_syscall_64+0x87/0x140
  [<0>] entry_SYSCALL_64_after_hwframe+0x76/0x7e

The cgroup freezing happens in the signal delivery path but
kvm_vm_worker_thread() thread never call into the signal delivery path while
joining non-root cgroups, so they never get frozen. Because the cgroup
freezer determines whether a given cgroup is frozen by comparing the number
of frozen threads to the total number of threads in the cgroup, the cgroup
never becomes frozen and users waiting for the state transition may hang
indefinitely.

There are two paths that we can take:

1. Make kvm_vm_worker_thread() call into signal delivery path.
   io_wq_worker() is in a similar boat and handles signal delivery and can
   be frozen and trapped like regular threads.

2. Keep the count of threads which can't be frozen per cgroup so that cgroup
   freezer can ignore these threads.

#1 is better in that the cgroup will actually be frozen when reported
frozen. However, the rather ambiguous criterion we've been using for cgroup
freezer is whether the cgroup can be safely snapshotted whil frozen and as
long as the workers not being frozen doesn't break that, we can go for #2
too.

What do you guys think?

Thanks.

-- 
tejun