[PATCH v1] memcg: fix soft lockup in the OOM process

[Chen Ridong <chenridong@huaweicloud.com>]

From: Chen Ridong <chenridong@huawei.com>

A soft lockup issue was found in the product with. About 56,000 tasks were
in the OOM cgroup, it was traversing them when the soft lockup was
triggered.

watchdog: BUG: soft lockup - CPU#2 stuck for 23s! [VM Thread:1503066]
CPU: 2 PID: 1503066 Comm: VM Thread Kdump: loaded Tainted: G
Hardware name: Huawei Cloud OpenStack Nova, BIOS
RIP: 0010:console_unlock+0x343/0x540
RSP: 0000:ffffb751447db9a0 EFLAGS: 00000247 ORIG_RAX: ffffffffffffff13
RAX: 0000000000000001 RBX: 0000000000000000 RCX: 00000000ffffffff
RDX: 0000000000000000 RSI: 0000000000000004 RDI: 0000000000000247
RBP: ffffffffafc71f90 R08: 0000000000000000 R09: 0000000000000040
R10: 0000000000000080 R11: 0000000000000000 R12: ffffffffafc74bd0
R13: ffffffffaf60a220 R14: 0000000000000247 R15: 0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f2fe6ad91f0 CR3: 00000004b2076003 CR4: 0000000000360ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 vprintk_emit+0x193/0x280
 printk+0x52/0x6e
 dump_task+0x114/0x130
 mem_cgroup_scan_tasks+0x76/0x100
 dump_header+0x1fe/0x210
 oom_kill_process+0xd1/0x100
 out_of_memory+0x125/0x570
 mem_cgroup_out_of_memory+0xb5/0xd0
 try_charge+0x720/0x770
 mem_cgroup_try_charge+0x86/0x180
 mem_cgroup_try_charge_delay+0x1c/0x40
 do_anonymous_page+0xb5/0x390
 handle_mm_fault+0xc4/0x1f0

This is because thousands of processes are in the OOM cgroup, it takes a
long time to traverse all of them. As a result, this lead to soft lockup
in the OOM process.

To fix this issue, add 'cond_resched' in the 'dump_task' function.

Fixes: 9cbb78bb3143 ("mm, memcg: introduce own oom handler to iterate only over its own threads")
Signed-off-by: Chen Ridong <chenridong@huawei.com>
---
 mm/oom_kill.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 1c485beb0b93..14260381cccc 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -390,6 +390,7 @@ static int dump_task(struct task_struct *p, void *arg)
 	if (!is_memcg_oom(oc) && !oom_cpuset_eligible(p, oc))
 		return 0;
 
+	cond_resched();
 	task = find_lock_task_mm(p);
 	if (!task) {
 		/*
-- 
2.34.1