[PATCH v3] mm: memcg: Fix memcg reclaim soft lockup

[Xunlei Pang]

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.

Acked-by: Chris Down <chris@xxxxxxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
Signed-off-by: Xunlei Pang <xlpang@xxxxxxxxxxxxxxxxx>
---
 mm/vmscan.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 99e1796..9727dd8 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2615,6 +2615,14 @@ static void shrink_node_memcgs(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();
+
 		mem_cgroup_calculate_protection(target_memcg, memcg);
 
 		if (mem_cgroup_below_min(memcg)) {
-- 
1.8.3.1