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

[Johannes Weiner]

On Wed, Aug 26, 2020 at 09:47:02PM +0800, 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.
> 
> Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
> Signed-off-by: Xunlei Pang <xlpang@xxxxxxxxxxxxxxxxx>

This generally makes sense to me but really should have a comment:

	/*
	 * This loop can become CPU-bound when there are thousands
	 * of cgroups that aren't eligible for reclaim - either
	 * because they don't have any pages, or because their
	 * memory is explicitly protected. Avoid soft lockups.
	 */
	 cond_resched();

The placement in the middle of the multi-part protection checks is a
bit odd too. It would be better to have it either at the top of the
loop, or at the end, by replacing the continues with goto next.