Re: [PATCH v2] mm: memcg: Fix memcg reclaim soft lockup

[Michal Hocko <mhocko@suse.com>]

On Wed 26-08-20 21:47:02, Xunlei Pang wrote:
> We've met softlockup with "CONFIG_PREEMPT_NONE=y", when
> the target memcg doesn't have any reclaimable memory.
> 
> It can be easily reproduced as below:
>  watchdog: BUG: soft lockup - CPU#0 stuck for 111s![memcg_test:2204]
>  CPU: 0 PID: 2204 Comm: memcg_test Not tainted 5.9.0-rc2+ #12
>  Call Trace:
>   shrink_lruvec+0x49f/0x640
>   shrink_node+0x2a6/0x6f0
>   do_try_to_free_pages+0xe9/0x3e0
>   try_to_free_mem_cgroup_pages+0xef/0x1f0
>   try_charge+0x2c1/0x750
>   mem_cgroup_charge+0xd7/0x240
>   __add_to_page_cache_locked+0x2fd/0x370
>   add_to_page_cache_lru+0x4a/0xc0
>   pagecache_get_page+0x10b/0x2f0
>   filemap_fault+0x661/0xad0
>   ext4_filemap_fault+0x2c/0x40
>   __do_fault+0x4d/0xf9
>   handle_mm_fault+0x1080/0x1790
> 
> It only happens on our 1-vcpu instances, because there's no chance
> for oom reaper to run to reclaim the to-be-killed process.
> 
> Add cond_resched() at the upper shrink_node_memcgs() to solve this
> issue, and any other possible issue like meomry.min protection.

I would just add
"
This will mean that we will get a scheduling point for each memcg in the
reclaimed hierarchy without any dependency on the reclaimable memory in
that memcg thus making it more predictable.
" 
> Suggested-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>

Acked-by: Michal Hocko <mhocko@suse.com>

Thanks!

> ---
>  mm/vmscan.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 99e1796..bbdc38b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2617,6 +2617,8 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>  
>  		mem_cgroup_calculate_protection(target_memcg, memcg);
>  
> +		cond_resched();
> +
>  		if (mem_cgroup_below_min(memcg)) {
>  			/*
>  			 * Hard protection.
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs