Re: [PATCH] memcg: avoid dead loop when setting memory.max

[Michal Hocko <mhocko@suse.com>]

On Tue 11-02-25 08:18:19, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.com>
> 
> A softlockup issue was found with stress test:
>  watchdog: BUG: soft lockup - CPU#27 stuck for 26s! [migration/27:181]
>  CPU: 27 UID: 0 PID: 181 Comm: migration/27 6.14.0-rc2-next-20250210 #1
>  Stopper: multi_cpu_stop <- stop_machine_from_inactive_cpu
>  RIP: 0010:stop_machine_yield+0x2/0x10
>  RSP: 0000:ff4a0dcecd19be48 EFLAGS: 00000246
>  RAX: ffffffff89c0108f RBX: ff4a0dcec03afe44 RCX: 0000000000000000
>  RDX: ff1cdaaf6eba5808 RSI: 0000000000000282 RDI: ff1cda80c1775a40
>  RBP: 0000000000000001 R08: 00000011620096c6 R09: 7fffffffffffffff
>  R10: 0000000000000001 R11: 0000000000000100 R12: ff1cda80c1775a40
>  R13: 0000000000000000 R14: 0000000000000001 R15: ff4a0dcec03afe20
>  FS:  0000000000000000(0000) GS:ff1cdaaf6eb80000(0000)
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000000000000000 CR3: 00000025e2c2a001 CR4: 0000000000773ef0
>  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>  PKRU: 55555554
>  Call Trace:
>   multi_cpu_stop+0x8f/0x100
>   cpu_stopper_thread+0x90/0x140
>   smpboot_thread_fn+0xad/0x150
>   kthread+0xc2/0x100
>   ret_from_fork+0x2d/0x50
> 
> The stress test involves CPU hotplug operations and memory control group
> (memcg) operations. The scenario can be described as follows:
> 
>  echo xx > memory.max 	cache_ap_online			oom_reaper
>  (CPU23)						(CPU50)
>  xx < usage		stop_machine_from_inactive_cpu
>  for(;;)			// all active cpus
>  trigger OOM		queue_stop_cpus_work
>  // waiting oom_reaper
>  			multi_cpu_stop(migration/xx)
>  			// sync all active cpus ack
>  			// waiting cpu23 ack
>  			// CPU50 loops in multi_cpu_stop
>  							waiting cpu50
> 
> Detailed explanation:
> 1. When the usage is larger than xx, an OOM may be triggered. If the
>    process does not handle with ths kill signal immediately, it will loop
>    in the memory_max_write.

Do I get it right that the issue is that mem_cgroup_out_of_memory which
doesn't have any cond_resched so it cannot yield to stopped kthread?
oom itself cannot make any progress because the oom victim is blocked as
per 3).

> 2. When cache_ap_online is triggered, the multi_cpu_stop is queued to the
>    active cpus. Within the multi_cpu_stop function,  it attempts to
>    synchronize the CPU states. However, the CPU23 didn't acknowledge
>    because it is stuck in a loop within the for(;;).
> 3. The oom_reaper process is blocked because CPU50 is in a loop, waiting
>    for CPU23 to acknowledge the synchronization request.
> 4. Finally, it formed cyclic dependency and lead to softlockup and dead
>    loop.
> 
> To fix this issue, add cond_resched() in the memory_max_write, so that
> it will not block migration task.

My first question was why this is not a problem in other
allocation/charge paths but this one is different because it doesn't
ever try to reclaim after MAX_RECLAIM_RETRIES reclaim rounds.
We do have scheduling points in the reclaim path which are no longer
triggered after we hit oom situation in this case.

I was thinking about having a guranteed cond_resched when oom killer
fails to find a victim but it seems the simplest fix for this particular
corner case is to add cond_resched as you did here. Hopefully we will
get rid of it very soon when !PREEMPT is removed.

Btw. this could be a problem on a single CPU machine even without CPU
hotplug as the oom repear won't run until memory_max_write yields the
cpu.

> Fixes: b6e6edcfa405 ("mm: memcontrol: reclaim and OOM kill when shrinking memory.max below usage")
> Signed-off-by: Chen Ridong <chenridong@huawei.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  mm/memcontrol.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 8d21c1a44220..16f3bdbd37d8 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4213,6 +4213,7 @@ static ssize_t memory_max_write(struct kernfs_open_file *of,
>  		memcg_memory_event(memcg, MEMCG_OOM);
>  		if (!mem_cgroup_out_of_memory(memcg, GFP_KERNEL, 0))
>  			break;
> +		cond_resched();
>  	}
>  
>  	memcg_wb_domain_size_changed(memcg);
> -- 
> 2.34.1

-- 
Michal Hocko
SUSE Labs