On Tue 11-02-25 08:18:19, Chen Ridong wrote:
> From: Chen Ridong <chenridong@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> 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
