Message-ID: <8c1ccd1b-47cd-43b6-b961-2829a5a24513@huaweicloud.com>
Date: Thu, 22 Aug 2024 08:57:47 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: martin.lau@...ux.dev, ast@...nel.org, daniel@...earbox.net,
 andrii@...nel.org, eddyz87@...il.com, song@...nel.org,
 yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org,
 sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org, tj@...nel.org,
 lizefan.x@...edance.com, hannes@...xchg.org, roman.gushchin@...ux.dev,
 Michal Koutný <mkoutny@...e.com>
Cc: bpf@...r.kernel.org, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/1] cgroup: fix deadlock caused by cgroup_mutex and
 cpu_hotplug_lock



On 2024/8/17 17:33, 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 pressuse test:
> 1. A large number of cpuset cgroups are deleted.
> 2. Set cpu on and off repeatly.
> 3. Set watchdog_thresh repeatly.
> The scripts can be obtained at LINK mentioned above the signature.
> 
> 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 cpusets are deleted asynchronously, which puts a
>     large number of cgroup_bpf_release works into system_wq. The max_active
>     of system_wq is WQ_DFL_ACTIVE(256). Consequently, all active works are
>     cgroup_bpf_release works, and many cgroup_bpf_release works will be put
>     into inactive queue. As illustrated in the diagram, there are 256 (in
>     the acvtive queue) + n (in the inactive queue) works.
> 2. Setting watchdog_thresh will hold cpu_hotplug_lock.read and put
>     smp_call_on_cpu work into system_wq. However step 1 has already filled
>     system_wq, 'sscs.work' is put into inactive queue. 'sscs.work' has
>     to wait until the works that were put into the inacvtive queue earlier
>     have executed (n cgroup_bpf_release), so it will be blocked for a while.
> 3. Cpu offline requires cpu_hotplug_lock.write, which is blocked by step 2.
> 4. Cpusets that were deleted at step 1 put cgroup_release works into
>     cgroup_destroy_wq. They are competing to get cgroup_mutex all the time.
>     When cgroup_metux is acqured by work at css_killed_work_fn, it will
>     call cpuset_css_offline, which needs to acqure cpu_hotplug_lock.read.
>     However, cpuset_css_offline will be blocked for step 3.
> 5. At this moment, there are 256 works in active queue that are
>     cgroup_bpf_release, they are attempting to acquire cgroup_mutex, and as
>     a result, all of them are blocked. Consequently, sscs.work can not be
>     executed. Ultimately, this situation leads to four processes being
>     blocked, forming a deadlock.
> 
> system_wq(step1)		WatchDog(step2)			cpu offline(step3)	cgroup_destroy_wq(step4)
> ...
> 2000+ cgroups deleted asyn
> 256 actives + n inactives
> 				__lockup_detector_reconfigure
> 				P(cpu_hotplug_lock.read)
> 				put sscs.work into system_wq
> 256 + n + 1(sscs.work)
> sscs.work wait to be executed
> 				warting sscs.work finish
> 								percpu_down_write
> 								P(cpu_hotplug_lock.write)
> 								...blocking...
> 											css_killed_work_fn
> 											P(cgroup_mutex)
> 											cpuset_css_offline
> 											P(cpu_hotplug_lock.read)
> 											...blocking...
> 256 cgroup_bpf_release
> mutex_lock(&cgroup_mutex);
> ..blocking...
> 
> 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@huawei.com/T/#t
> Signed-off-by: Chen Ridong <chenridong@...wei.com>
> ---
>   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 c964dd7ff967..17ac19bc8106 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 75058fbf4450..77fa9ed69c86 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,

Ping.
Hi,TJ, Roman and Michal, I have updated commit message, I think it can 
be much clearer now, can you review it again?

Thanks,
Ridong

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