[Devel] [PATCH RHEL7 COMMIT] mm: memcg: fix memcg reclaim soft lockup

[Vasily Averin]

The commit is pushed to "branch-rh7-3.10.0-1160.11.1.vz7.172.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1160.11.1.vz7.172.11
------>
commit b79594a04564af843d953594f0534abd6679cd48
Author: Xunlei Pang <xlpang at linux.alibaba.com>
Date:   Wed Feb 3 10:39:59 2021 +0300

    mm: memcg: fix memcg reclaim soft lockup
    
    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>
    Reviewed-by: Kirill Tkhai <ktkhai at virtuozzo.com>
---
 mm/vmscan.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 240435e..ebb97ad 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2678,6 +2678,14 @@ 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;