On 2024/7/24 19:08, Hillf Danton wrote:
On Fri, 19 Jul 2024 02:52:32 +0000 Chen Ridong <chenridong@xxxxxxxxxx>
We found a hung_task problem as shown below:
INFO: task kworker/0:0:8 blocked for more than 327 seconds.
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/0:0 state:D stack:13920 pid:8 ppid:2 flags:0x00004000
Workqueue: events cgroup_bpf_release
Call Trace:
<TASK>
__schedule+0x5a2/0x2050
? find_held_lock+0x33/0x100
? wq_worker_sleeping+0x9e/0xe0
schedule+0x9f/0x180
schedule_preempt_disabled+0x25/0x50
__mutex_lock+0x512/0x740
? cgroup_bpf_release+0x1e/0x4d0
? cgroup_bpf_release+0xcf/0x4d0
? process_scheduled_works+0x161/0x8a0
? cgroup_bpf_release+0x1e/0x4d0
? mutex_lock_nested+0x2b/0x40
? __pfx_delay_tsc+0x10/0x10
mutex_lock_nested+0x2b/0x40
cgroup_bpf_release+0xcf/0x4d0
? process_scheduled_works+0x161/0x8a0
? trace_event_raw_event_workqueue_execute_start+0x64/0xd0
? process_scheduled_works+0x161/0x8a0
process_scheduled_works+0x23a/0x8a0
worker_thread+0x231/0x5b0
? __pfx_worker_thread+0x10/0x10
kthread+0x14d/0x1c0
? __pfx_kthread+0x10/0x10
ret_from_fork+0x59/0x70
? __pfx_kthread+0x10/0x10
ret_from_fork_asm+0x1b/0x30
</TASK>
This issue can be reproduced by the following methods:
1. A large number of cpuset cgroups are deleted.
2. Set cpu on and off repeatly.
3. Set watchdog_thresh repeatly.
The reason for this issue is cgroup_mutex and cpu_hotplug_lock are
acquired in different tasks, which may lead to deadlock.
It can lead to a deadlock through the following steps:
1. A large number of cgroups are deleted, which will put a large
number of cgroup_bpf_release works into system_wq. The max_active
of system_wq is WQ_DFL_ACTIVE(256). When cgroup_bpf_release can not
get cgroup_metux, it may cram system_wq, and it will block work
enqueued later.
2. Setting watchdog_thresh will hold cpu_hotplug_lock.read and put
smp_call_on_cpu work into system_wq. However it may be blocked by
step 1.
3. Cpu offline requires cpu_hotplug_lock.write, which is blocked by step 2.
4. When a cpuset is deleted, cgroup release work is placed on
cgroup_destroy_wq, it will hold cgroup_metux and acquire
cpu_hotplug_lock.read. Acquiring cpu_hotplug_lock.read is blocked by
cpu_hotplug_lock.write as mentioned by step 3. Finally, it forms a
loop and leads to a deadlock.
cgroup_destroy_wq(step4) cpu offline(step3) WatchDog(step2) system_wq(step1)
......
__lockup_detector_reconfigure:
P(cpu_hotplug_lock.read)
...
...
percpu_down_write:
P(cpu_hotplug_lock.write)
...256+ works
cgroup_bpf_release:
P(cgroup_mutex)
smp_call_on_cpu:
Wait system_wq
...
css_killed_work_fn:
P(cgroup_mutex)
...
cpuset_css_offline:
P(cpu_hotplug_lock.read)
worker_thread()
manage_workers()
maybe_create_worker()
create_worker() // has nothing to do with WQ_DFL_ACTIVE
process_scheduled_works()
Given idle worker created independent of WQ_DFL_ACTIVE before handling
work item, no deadlock could rise in your scenario above.
Hello Hillf, did you mean to say this issue couldn't happen?
I wish it hadn't happen, as it took me a long time to figure out.
However, it did happen. It could be reproduced with the method I
offered, You can access the scripts using this link:
https://lore.kernel.org/cgroups/e90c32d2-2a85-4f28-9154-09c7d320cb60@xxxxxxxxxx/T/#t.
It's not about how the pool's workers were created, but rather the
limit(system_wq ) of workqueue. If system_wq reaches its max_active
limit, the work enqueued afterward will be placed on
pwq->inactive_works. In this scenario described above, the problem is
all active works(cgroup_bpf_release) are blocked, and the
inactive_works(smp_call_on_cpu) couldn't be executed when it forms a loop.
We have discussed this before in V1, you can find in the Link.
Thanks
Ridong
