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Date:   Wed, 18 Mar 2020 10:42:19 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     David Rientjes <rientjes@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Vlastimil Babka <vbabka@suse.cz>,
        Robert Kolchmeyer <rkolchmeyer@google.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [patch v2] mm, oom: prevent soft lockup on memcg oom for UP
 systems
Message-ID: <20200318094219.GE21362@dhcp22.suse.cz>
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On Tue 17-03-20 17:55:04, David Rientjes 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.
"
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.
"
> 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