[Devel] [PATCH rh7 1/2] mm: memcg: fix memcg reclaim soft lockup

[Andrey Ryabinin]

From: Xunlei Pang <xlpang at linux.alibaba.com>

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.

Suggested-by: Michal Hocko <mhocko at suse.com>
Signed-off-by: Xunlei Pang <xlpang at linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Acked-by: Chris Down <chris at chrisdown.name>
Acked-by: Michal Hocko <mhocko at suse.com>
Acked-by: Johannes Weiner <hannes at cmpxchg.org>
Link: http://lkml.kernel.org/r/1598495549-67324-1-git-send-email-xlpang@linux.alibaba.com
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

https://jira.sw.ru/browse/PSBM-125095
(cherry picked from commit e3336cab2579012b1e72b5265adf98e2d6e244ad)
Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 mm/vmscan.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 85622f235e78..080500f4e366 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2684,6 +2684,14 @@ static void shrink_zone(struct zone *zone, struct scan_control *sc,
 		do {
 			unsigned long lru_pages, 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();
+
 			if (!sc->may_thrash && mem_cgroup_low(root, memcg))
 				continue;
 
-- 
2.26.2