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

Michal Hocko
2020-08-26T17:30:09Z

On Wed 26-08-20 12:43:32, Johannes Weiner wrote:
> 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@suse.com>
> > Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>
>
> 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.

Yes makes sense. I would stick it to the begining of the loop to make it
stand out and make it obvious wrt code flow.

--
Michal Hocko
SUSE Labs