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@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. 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
