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Wed, 06 Nov 2019 18:21:59 -0800 (PST) MIME-Version: 1.0 References: <20191106225131.3543616-1-guro@fb.com> <20191107002204.GA96548@cmpxchg.org> <20191107014307.GA1158@castle.dhcp.thefacebook.com> In-Reply-To: <20191107014307.GA1158@castle.dhcp.thefacebook.com> From: Shakeel Butt Date: Wed, 6 Nov 2019 18:21:48 -0800 Message-ID: Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm() To: Roman Gushchin Cc: Johannes Weiner , Linux MM , Andrew Morton , Michal Hocko , LKML , Kernel Team , "stable@vger.kernel.org" , Tejun Heo Content-Type: text/plain; charset="UTF-8" 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 6, 2019 at 5:43 PM Roman Gushchin wrote: > > On Wed, Nov 06, 2019 at 05:25:26PM -0800, Shakeel Butt wrote: > > On Wed, Nov 6, 2019 at 4:22 PM Johannes Weiner wrote: > > > > > > 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: Johannes Weiner > > > > > > 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. > > > > Should get_mem_cgroup_from_current() and get_mem_cgroup_from_page() be > > switched to css_tryget() as well then? > > In those case it can't cause a rcu stall, so it's not a so urgent. > But you are right, we should probably do the same here. I'll look > at all remaining callers and prepare the patchset. > > I'll also probably rename it to css_tryget_if_online() to make obvious > that it doesn't hold the cgroup from being onlined. > SGTM