Re: [PATCH stable v5.4] mm: memcg: fix memcg reclaim soft lockup

[Greg KH]

On Mon, Sep 21, 2020 at 11:05:08AM -0700, Julius Hemanth Pitti wrote:
> From: Xunlei Pang <xlpang@xxxxxxxxxxxxxxxxx>
> 
> commit e3336cab2579012b1e72b5265adf98e2d6e244ad upstream
> 
> 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 a cond_resched() at the upper shrink_node_memcgs() to solve this
> issue, 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.
> 
> [jpitti@xxxxxxxxx:
>    - backported to v5.4.y
>    - Upstream patch applies fix in shrink_node_memcgs(), which
>      is not present to v5.4.y. Appled to shrink_node()]

Thanks for this, now queued up here and for 4.19

greg k-h