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From:   Roman Gushchin <guro@fb.com>
To:     Michal Hocko <mhocko@kernel.org>
CC:     "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in
 get_mem_cgroup_from_mm()
Thread-Topic: [PATCH 1/2] mm: memcg: switch to css_tryget() in
 get_mem_cgroup_from_mm()
Thread-Index: AQHVlPTSqR6tDyqEq0WgIBYYtGCX/Kd/odWAgABI+QA=
Date:   Thu, 7 Nov 2019 16:42:41 +0000
Message-ID: <20191107164236.GB2919@castle.dhcp.thefacebook.com>
References: <20191106225131.3543616-1-guro@fb.com>
 <20191107122125.GS8314@dhcp22.suse.cz>
In-Reply-To: <20191107122125.GS8314@dhcp22.suse.cz>
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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():
> >=20
> >  rcu: INFO: rcu_sched self-detected stall on CPU
> >  rcu: 33-....: (21000 ticks this GP) idle=3D6c6/1/0x4000000000000002 so=
ftirq=3D35441/35441 fqs=3D5017
> >  (t=3D21031 jiffies g=3D324821 q=3D95837) 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
> >=20
> > The problem is caused by an exiting task which is associated with
> > an offline memcg.
>=20
> 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 st=
ages
of life.