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charset=us-ascii Content-Disposition: inline In-Reply-To: <20191106225131.3543616-1-guro@fb.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 > Cc: stable@vger.kernel.org > Cc: Tejun Heo Acked-by: Tejun Heo Thanks. -- tejun