Re: [patch] mm, oom: prevent soft lockup on memcg oom for UP systems

[Michal Hocko]

On Tue 10-03-20 14:39:48, 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.
> 
> 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 something ends up actually yielding the processor back to
> the victim to allow for memory freeing.  Most appropriate place appears to
> be shrink_node_memcgs() where the iteration of all decendant memcgs could
> be particularly lengthy.

There is a cond_resched in shrink_lruvec and another one in
shrink_page_list. Why doesn't any of them hit? Is it because there are
no pages on the LRU list? Because rss data suggests there should be
enough pages to go that path. Or maybe it is shrink_slab path that takes
too long?

The patch itself makes sense to me but I would like to see more
explanation on how that happens.

Thanks.

> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
>  mm/vmscan.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2637,6 +2637,8 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>  		unsigned long reclaimed;
>  		unsigned long scanned;
>  
> +		cond_resched();
> +
>  		switch (mem_cgroup_protected(target_memcg, memcg)) {
>  		case MEMCG_PROT_MIN:
>  			/*

-- 
Michal Hocko
SUSE Labs