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.
