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SubjectRe: [patch v2] mm, oom: prevent soft lockup on memcg oom for UP systems
On Wed, 18 Mar 2020, Michal Hocko wrote:

> > When a process is oom killed as a result of memcg limits and the victim
> > is waiting to exit, nothing ends up actually yielding the processor back
> > to the victim on UP systems with preemption disabled. Instead, the
> > charging process simply loops in memcg reclaim and eventually soft
> > lockups.
>
> It seems that my request to describe the setup got ignored. Sigh.
>
> > Memory cgroup out of memory: Killed process 808 (repro) total-vm:41944kB,
> > anon-rss:35344kB, file-rss:504kB, shmem-rss:0kB, UID:0 pgtables:108kB
> > oom_score_adj:0
> > watchdog: BUG: soft lockup - CPU#0 stuck for 23s! [repro:806]
> > CPU: 0 PID: 806 Comm: repro Not tainted 5.6.0-rc5+ #136
> > RIP: 0010:shrink_lruvec+0x4e9/0xa40
> > ...
> > Call Trace:
> > shrink_node+0x40d/0x7d0
> > do_try_to_free_pages+0x13f/0x470
> > try_to_free_mem_cgroup_pages+0x16d/0x230
> > try_charge+0x247/0xac0
> > mem_cgroup_try_charge+0x10a/0x220
> > mem_cgroup_try_charge_delay+0x1e/0x40
> > handle_mm_fault+0xdf2/0x15f0
> > do_user_addr_fault+0x21f/0x420
> > page_fault+0x2f/0x40
> >
> > Make sure that once the oom killer has been called that we forcibly yield
> > if current is not the chosen victim regardless of priority to allow for
> > memory freeing. The same situation can theoretically occur in the page
> > allocator, so do this after dropping oom_lock there as well.
>
> I would have prefered the cond_resched solution proposed previously but
> I can live with this as well. I would just ask to add more information
> to the changelog. E.g.

I'm still planning on sending the cond_resched() change as well, but not
as advertised to fix this particular issue per Tetsuo's feedback. I think
the reported issue showed it's possible to excessively loop in reclaim
without a conditional yield depending on various memcg configs and the
shrink_node_memcgs() cond_resched() is still appropriate for interactivity
but also because the iteration of memcgs can be particularly long.

> "
> We used to have a short sleep after the oom handling but 9bfe5ded054b
> ("mm, oom: remove sleep from under oom_lock") has removed it because
> sleep inside the oom_lock is dangerous. This patch restores the sleep
> outside of the lock.

Will do.

> "
> > Suggested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
> > Tested-by: Robert Kolchmeyer <rkolchmeyer@google.com>
> > Cc: stable@vger.kernel.org
> > Signed-off-by: David Rientjes <rientjes@google.com>
> > ---
> > mm/memcontrol.c | 2 ++
> > mm/page_alloc.c | 2 ++
> > 2 files changed, 4 insertions(+)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1576,6 +1576,8 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
> > */
> > ret = should_force_charge() || out_of_memory(&oc);
> > mutex_unlock(&oom_lock);
> > + if (!fatal_signal_pending(current))
> > + schedule_timeout_killable(1);
>
> Check for fatal_signal_pending is redundant.
>
> --
> Michal Hocko
> SUSE Labs
>

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