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SubjectRe: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()
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On Thu, Nov 07, 2019 at 01:21:25PM +0100, Michal Hocko wrote:
> 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?

It's an exiting task with the PF_EXITING flag set and it's in their late stages
of life.

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