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.
