On 2024/7/19 10:52, Chen Ridong wrote:
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)
To fix the problem, place cgroup_bpf_release works on cgroup_destroy_wq,
which can break the loop and solve the problem. System wqs are for misc
things which shouldn't create a large number of concurrent work items.
If something is going to generate >WQ_DFL_ACTIVE(256) concurrent work
items, it should use its own dedicated workqueue.
Fixes: 4bfc0bb2c60e ("bpf: decouple the lifetime of cgroup_bpf from cgroup itself")
Link: https://lore.kernel.org/cgroups/e90c32d2-2a85-4f28-9154-09c7d320cb60@xxxxxxxxxx/T/#t
Signed-off-by: Chen Ridong <chenridong@xxxxxxxxxx>
---
kernel/bpf/cgroup.c | 2 +-
kernel/cgroup/cgroup-internal.h | 1 +
kernel/cgroup/cgroup.c | 2 +-
3 files changed, 3 insertions(+), 2 deletions(-)
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 8ba73042a239..a611a1274788 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -334,7 +334,7 @@ static void cgroup_bpf_release_fn(struct percpu_ref *ref)
struct cgroup *cgrp = container_of(ref, struct cgroup, bpf.refcnt);
INIT_WORK(&cgrp->bpf.release_work, cgroup_bpf_release);
- queue_work(system_wq, &cgrp->bpf.release_work);
+ queue_work(cgroup_destroy_wq, &cgrp->bpf.release_work);
}
/* Get underlying bpf_prog of bpf_prog_list entry, regardless if it's through
diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index 520b90dd97ec..9e57f3e9316e 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -13,6 +13,7 @@
extern spinlock_t trace_cgroup_path_lock;
extern char trace_cgroup_path[TRACE_CGROUP_PATH_LEN];
extern void __init enable_debug_cgroup(void);
+extern struct workqueue_struct *cgroup_destroy_wq;
/*
* cgroup_path() takes a spin lock. It is good practice not to take
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index e32b6972c478..3317e03fe2fb 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -124,7 +124,7 @@ DEFINE_PERCPU_RWSEM(cgroup_threadgroup_rwsem);
* destruction work items don't end up filling up max_active of system_wq
* which may lead to deadlock.
*/
-static struct workqueue_struct *cgroup_destroy_wq;
+struct workqueue_struct *cgroup_destroy_wq;
/* generate an array of cgroup subsystem pointers */
#define SUBSYS(_x) [_x ## _cgrp_id] = &_x ## _cgrp_subsys,
Friendly ping.
