Re: [PATCH v3] memcg: fix soft lockup in the OOM process

[Vlastimil Babka <vbabka@suse.cz>]

On 12/24/24 03:52, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.com>

+CC RCU

> 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, call 'cond_resched' in the 'mem_cgroup_scan_tasks'
> function per 1000 iterations. For global OOM, call
> 'touch_softlockup_watchdog' per 1000 iterations to avoid this issue.
> 
> Fixes: 9cbb78bb3143 ("mm, memcg: introduce own oom handler to iterate only over its own threads")
> Signed-off-by: Chen Ridong <chenridong@huawei.com>
> ---
>  mm/memcontrol.c | 7 ++++++-
>  mm/oom_kill.c   | 8 +++++++-
>  2 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 65fb5eee1466..46f8b372d212 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1161,6 +1161,7 @@ void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
>  {
>  	struct mem_cgroup *iter;
>  	int ret = 0;
> +	int i = 0;
>  
>  	BUG_ON(mem_cgroup_is_root(memcg));
>  
> @@ -1169,8 +1170,12 @@ void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
>  		struct task_struct *task;
>  
>  		css_task_iter_start(&iter->css, CSS_TASK_ITER_PROCS, &it);
> -		while (!ret && (task = css_task_iter_next(&it)))
> +		while (!ret && (task = css_task_iter_next(&it))) {
> +			/* Avoid potential softlockup warning */
> +			if ((++i & 1023) == 0)
> +				cond_resched();
>  			ret = fn(task, arg);
> +		}
>  		css_task_iter_end(&it);
>  		if (ret) {
>  			mem_cgroup_iter_break(memcg, iter);
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 1c485beb0b93..044ebab2c941 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -44,6 +44,7 @@
>  #include <linux/init.h>
>  #include <linux/mmu_notifier.h>
>  #include <linux/cred.h>
> +#include <linux/nmi.h>
>  
>  #include <asm/tlb.h>
>  #include "internal.h"
> @@ -430,10 +431,15 @@ static void dump_tasks(struct oom_control *oc)
>  		mem_cgroup_scan_tasks(oc->memcg, dump_task, oc);
>  	else {
>  		struct task_struct *p;
> +		int i = 0;
>  
>  		rcu_read_lock();
> -		for_each_process(p)
> +		for_each_process(p) {
> +			/* Avoid potential softlockup warning */
> +			if ((++i & 1023) == 0)
> +				touch_softlockup_watchdog();

This might suppress the soft lockup, but won't a rcu stall still be detected?

>  			dump_task(p, oc);
> +		}
>  		rcu_read_unlock();
>  	}
>  }