From: Xunlei Pang xlpang@linux.alibaba.com

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@cisco.com: - backported to v4.19.y - Upstream patch applies fix in shrink_node_memcgs(), which is not present to v4.19.y. Appled to shrink_node()]

Suggested-by: Michal Hocko mhocko@suse.com Signed-off-by: Xunlei Pang xlpang@linux.alibaba.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Acked-by: Chris Down chris@chrisdown.name Acked-by: Michal Hocko mhocko@suse.com Acked-by: Johannes Weiner hannes@cmpxchg.org Link: http://lkml.kernel.org/r/1598495549-67324-1-git-send-email-xlpang@linux.alib... Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Fixes: b0dedc49a2da ("mm/vmscan.c: iterate only over charged shrinkers during memcg shrink_slab()") Cc: stable@vger.kernel.org Signed-off-by: Julius Hemanth Pitti jpitti@cisco.com --- mm/vmscan.c | 8 ++++++++ 1 file changed, 8 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c index bc2ecd43251a..b93dc8fc6007 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -2708,6 +2708,14 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc) unsigned long reclaimed; unsigned long scanned;

+ /* + * This loop can become CPU-bound when target memcgs + * aren't eligible for reclaim - either because they + * don't have any reclaimable pages, or because their + * memory is explicitly protected. Avoid soft lockups. + */ + cond_resched(); + switch (mem_cgroup_protected(root, memcg)) { case MEMCG_PROT_MIN: /*