Re: [PATCH v2] workqueue: Use active mask for new worker when pool is DISASSOCIATED

[Lai Jiangshan]

On Wed, Jul 13, 2022 at 8:51 PM Schspa Shi <schspa@xxxxxxxxx> wrote:
>
> When CPU-[un]hotplugs, all workers will be bound to active CPU via
> unbind_workers().
>
> But the unbound worker still has a chance to create a new worker, which
> has bound the newly created task to pool->attrs->cpumask. But the CPU has
> been unplugged.
>
> Please refer to the following scenarios.
>
>            CPU0                                  CPU1
> ------------------------------------------------------------------
> sched_cpu_deactivate(cpu_active_mask clear)
> workqueue_offline_cpu(work pool POOL_DISASSOCIATED)
>   -- all worker will migrate to another cpu --
>                                     worker_thread
>                                     -- will create new worker if
>                                        pool->worklist is not empty
>                                        create_worker()
>                                      -- new kworker will bound to CPU0
>                                (pool->attrs->cpumask will be mask of CPU0).
>       kworker/0:x will running on rq
>
> sched_cpu_dying
>   if (rq->nr_running != 1 || rq_has_pinned_tasks(rq))
>     WARN(true, "Dying CPU not properly vacated!");
>       ---------OOPS-------------
>
> And the crash log as fellowing.
>
> The crash log is as follows:
> [ 1622.829074] ------------[ cut here ]------------
> [ 1622.829081] Dying CPU not properly vacated!
> [ 1622.829091] WARNING: CPU: 3 PID: 31 at kernel/sched/core.c:7756 sched_cpu_dying+0x74/0x204
> [ 1622.829374] CPU: 3 PID: 31 Comm: migration/3 Tainted: P           O      5.10.59-rt52 #2
> [ 1622.829386] Stopper: multi_cpu_stop+0x0/0x160 <- 0x0
> [ 1622.829400] pstate: 60c00009 (nZCv daif +PAN +UAO -TCO BTYPE=--)
> [ 1622.829408] pc : sched_cpu_dying+0x74/0x204
> [ 1622.829415] lr : sched_cpu_dying+0x74/0x204
> [ 1622.829421] sp : ffff800012933c70
> [ 1622.829424] pmr_save: 00000060
> [ 1622.829426] x29: ffff800012933c70 x28: 0000000000000000
> [ 1622.829435] x27: 0000000000000000 x26: 0000000000000001
> [ 1622.829444] x25: 0000000000000000 x24: ffff800018353c2c
> [ 1622.829452] x23: 0000000000000003 x22: 0000000000000003
> [ 1622.829460] x21: 0000000000000059 x20: 0000000000000000
> [ 1622.829468] x19: ffff00027ee68a80 x18: 0000000000000000
> [ 1622.829477] x17: 0000000000000000 x16: 0000000000000000
> [ 1622.829485] x15: ffffffffffffffff x14: ffff80001169ae30
> [ 1622.829493] x13: ffffffffffc38c07 x12: ffffffffffffffff
> [ 1622.829501] x11: ffffffffffe00000 x10: ffff80001169ae58
> [ 1622.829510] x9 : 000000000000001e x8 : ffff80001169ae30
> [ 1622.829518] x7 : ffff800012933ab0 x6 : 00000000ffff0e20
> [ 1622.829526] x5 : ffff00027ee62988 x4 : 00000000ffff0e20
> [ 1622.829535] x3 : ffff800011e30180 x2 : 0000000100000002
> [ 1622.829543] x1 : 0000000000000000 x0 : 0000000000000000
> [ 1622.829552] Call trace:
> [ 1622.829555]  sched_cpu_dying+0x74/0x204
> [ 1622.829562]  cpuhp_invoke_callback+0xc0/0x1b0
> [ 1622.829571]  take_cpu_down+0xbc/0xd4
> [ 1622.829577]  multi_cpu_stop+0x138/0x160
> [ 1622.829584]  cpu_stopper_thread+0x9c/0x118
> [ 1622.829591]  smpboot_thread_fn+0x1e8/0x1ec
> [ 1622.829598]  kthread+0x114/0x124
> [ 1622.829606]  ret_from_fork+0x10/0x30
> [ 1622.829615] ---[ end trace 0000000000000002 ]---
> [ 1623.830273] CPU3 enqueued tasks (2 total):
> [ 1623.830291]  pid: 31, name: migration/3
> [ 1623.830440]  pid: 25654, name: kworker/3:0
> [ 1623.830444] task:kworker/3:0     state:R  running task     stack:    0 pid:25654 ppid:     2 flags:0x00000028
> [ 1623.830458] Call trace:
> [ 1623.830460]  __switch_to+0x164/0x17c
> [ 1623.830472]  __schedule+0x4cc/0x5c0
> [ 1623.830483]  schedule+0x7c/0xcc
> [ 1623.830491]  schedule_preempt_disabled+0x14/0x24
> [ 1623.830500]  kthread+0xd8/0x124
> [ 1623.830509]  ret_from_fork+0x10/0x30
>
> In the crash log, the error enqueued kworker(pid: 25654, name: kworker/3:0)
> is a newly created thread, and have bind to a unpluged CPU 3.
>
> Can crash dump can also verify this.
> crash> task -R nr_cpus_allowed,cpus_mask 25654
> PID: 25654  TASK: ffff000181ff0000  CPU: 3   COMMAND: "kworker/3:0"
>   nr_cpus_allowed = 1,
>   cpus_mask = {
>     bits = {8, 0}
>   },
>
> crash> struct worker_pool.cpu,nr_workers,attrs 0xffff00027ee68380
>   cpu = 3,
>   nr_workers = 3,
>   attrs = 0xffff000180004480,
> crash> struct workqueue_attrs 0xffff000180004480
> struct workqueue_attrs {
>   nice = 0,
>   cpumask = {{
>       bits = {8, 0}
>     }},
>   no_numa = false
> }
>
> The stack trace of the bad running task was dumped via the following patch:
> Link: https://lore.kernel.org/all/20220519161125.41144-1-schspa@xxxxxxxxx/
> And I think this debug patch needs to be added to the mainline,
> it can help us to debug this kind of problem
>
> To fix it, we can use cpu_active_mask when work pool is DISASSOCIATED.
>
> Signed-off-by: Schspa Shi <schspa@xxxxxxxxx>
>
> --
>
> Changelog:
> v1 -> v2:
>         - Move worker task bind to worker_attach_to_pool, remove extra
>         wq_pool_attach_mutex added.
>         - Add a timing diagram to make this question clearer.
> ---
>  kernel/workqueue.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 1ea50f6be843..3f6e6d884793 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1867,13 +1867,13 @@ static void worker_attach_to_pool(struct worker *worker,
>          * stable across this function.  See the comments above the flag
>          * definition for details.
>          */
> -       if (pool->flags & POOL_DISASSOCIATED)
> +       if (pool->flags & POOL_DISASSOCIATED) {
>                 worker->flags |= WORKER_UNBOUND;
> -       else
> +               set_cpus_allowed_ptr(worker->task, cpu_active_mask);

> +       } else {
>                 kthread_set_per_cpu(worker->task, pool->cpu);
> -
> -       if (worker->rescue_wq)
>                 set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
> +       }



For unbound pools (get_unbound_pool(), which also has POOL_DISASSOCIATED),
pool->attrs->cpumask should be used if pool->attrs->cpumask has active cpu.

         if (cpumask_intersects(pool->attrs->cpumask, cpu_active_mask))
                 set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
         else
                 set_cpus_allowed_ptr(worker->task, wq_unbound_cpumask);

And put it above the "if (pool->flags & POOL_DISASSOCIATED)" code block.

>
>         list_add_tail(&worker->node, &pool->workers);
>         worker->pool = pool;
> @@ -1952,7 +1952,6 @@ static struct worker *create_worker(struct worker_pool *pool)
>                 goto fail;
>
>         set_user_nice(worker->task, pool->attrs->nice);
> -       kthread_bind_mask(worker->task, pool->attrs->cpumask);

worker->flags |= PF_NO_SETAFFINITY;
PF_NO_SETAFFINITY needs to be set for workers.

>
>         /* successful, attach the worker to the pool */
>         worker_attach_to_pool(worker, pool);
> --
> 2.29.0
>