Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()

Michal Hocko
2019-11-07T12:21:31Z

On Wed 06-11-19 14:51:30, Roman Gushchin wrote:
> We've encountered a rcu stall in get_mem_cgroup_from_mm():
>
> rcu: INFO: rcu_sched self-detected stall on CPU
> rcu: 33-....: (21000 ticks this GP) idle=6c6/1/0x4000000000000002 softirq=35441/35441 fqs=5017
> (t=21031 jiffies g=324821 q=95837) NMI backtrace for cpu 33
> <...>
> RIP: 0010:get_mem_cgroup_from_mm+0x2f/0x90
> <...>
> __memcg_kmem_charge+0x55/0x140
> __alloc_pages_nodemask+0x267/0x320
> pipe_write+0x1ad/0x400
> new_sync_write+0x127/0x1c0
> __kernel_write+0x4f/0xf0
> dump_emit+0x91/0xc0
> writenote+0xa0/0xc0
> elf_core_dump+0x11af/0x1430
> do_coredump+0xc65/0xee0
> ? unix_stream_sendmsg+0x37d/0x3b0
> get_signal+0x132/0x7c0
> do_signal+0x36/0x640
> ? recalc_sigpending+0x17/0x50
> exit_to_usermode_loop+0x61/0xd0
> do_syscall_64+0xd4/0x100
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> The problem is caused by an exiting task which is associated with
> an offline memcg.

Hmm, how can we have a task in an offline memcg? I thought that any
existing task will prevent cgroup removal from proceeding. Is this some
sort of race where the task managed to disassociate from the cgroup
while there is still a binding to a memcg existing? What am I missing?

--
Michal Hocko
SUSE Labs