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Date:   Wed, 6 Nov 2019 16:22:04 -0800
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, 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()
Message-ID: <20191107002204.GA96548@cmpxchg.org>
References: <20191106225131.3543616-1-guro@fb.com>
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On Wed, Nov 06, 2019 at 02:51:30PM -0800, 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. We're iterating over and over in the
> do {} while (!css_tryget_online()) loop, but obviously the memcg won't
> become online and the exiting task won't be migrated to a live memcg.
> 
> Let's fix it by switching from css_tryget_online() to css_tryget().
> 
> As css_tryget_online() cannot guarantee that the memcg won't go
> offline, the check is usually useless, except some rare cases
> when for example it determines if something should be presented
> to a user.
> 
> A similar problem is described by commit 18fa84a2db0e ("cgroup: Use
> css_tryget() instead of css_tryget_online() in task_get_css()").
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Cc: stable@vger.kernel.org
> Cc: Tejun Heo <tj@kernel.org>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

The bug aside, it doesn't matter whether the cgroup is online for the
callers. It used to matter when offlining needed to evacuate all
charges from the memcg, and so needed to prevent new ones from showing
up, but we don't care now.