Re: [PATCH v3] workqueue: Use active mask for new worker when pool is DISASSOCIATED

[Zhaohui Shi]

Lai Jiangshan <jiangshanlai@xxxxxxxxx> writes:

> On Thu, Jul 14, 2022 at 11:16 AM 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
>
> How will the new kworker bound to CPU0?  Could you give more details?
>

It's because we use pool->attrs->cpumask for the newly created worker
to bind it to CPU0.

Please note that the CPU0 marked here does not correspond to the
fault log below, maybe I should change this CPU0 to CPU3, then it
can be correspond to the exception log below.


> Since the pool is POOL_DISASSOCIATED and kthread_is_per_cpu() will
> be false for the new worker. ttwu() will put it on a fallback CPU IIUC
> (see select_task_rq()).
>

It won't put it on a fallback CPU, is_cpu_allowed() will use
cpu_online_mask to check if a kthread can running on this CPU, we
need cpu_active_mask in this case.

see:
static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
{
	...
	/* KTHREAD_IS_PER_CPU is always allowed. */
	if (kthread_is_per_cpu(p))
		return cpu_online(cpu);
	...
}

>>                                (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-------------
>>
>
>
>> 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.
>
> use wq_unbound_cpumask.
>

Yes, I forgot to change this.

>>
>> Signed-off-by: Schspa Shi <schspa@xxxxxxxxx>
>
> Please solo CC Peter, as:
>
> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>

OK, thanks for reminding me.

>>
>> --
>>
>> 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.
>> v2 -> v3:
>>         - Add missing PF_NO_SETAFFINITY, use cpumask_intersects to
>>         avoid setting bad mask for unbound work pool as Lai Jiangshan
>>         advised.
>>         - Call kthread_set_pre_cpu correctly for unbound worker.
>> ---
>>  kernel/workqueue.c | 16 +++++++++++-----
>>  1 file changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
>> index 1ea50f6be843..b3e9289d9640 100644
>> --- a/kernel/workqueue.c
>> +++ b/kernel/workqueue.c
>> @@ -1860,8 +1860,16 @@ static struct worker *alloc_worker(int node)
>>  static void worker_attach_to_pool(struct worker *worker,
>>                                    struct worker_pool *pool)
>>  {
>> +       const struct cpumask *cpu_mask;
>> +
>>         mutex_lock(&wq_pool_attach_mutex);
>>
>> +       if (cpumask_intersects(pool->attrs->cpumask, cpu_active_mask))
>> +               cpu_mask = pool->attrs->cpumask;
>> +       else
>> +               cpu_mask = wq_unbound_cpumask;
>> +
>> +       set_cpus_allowed_ptr(worker->task, cpu_mask);
>>         /*
>>          * The wq_pool_attach_mutex ensures %POOL_DISASSOCIATED remains
>>          * stable across this function.  See the comments above the flag
>> @@ -1870,10 +1878,8 @@ static void worker_attach_to_pool(struct worker *worker,
>>         if (pool->flags & POOL_DISASSOCIATED)
>>                 worker->flags |= WORKER_UNBOUND;
>>         else
>> -               kthread_set_per_cpu(worker->task, pool->cpu);
>> -
>> -       if (worker->rescue_wq)
>> -               set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
>> +               kthread_set_per_cpu(worker->task,
>> +                               cpu_mask == wq_unbound_cpumask ? -1 : pool->cpu);
>
> Only workers for percpu pool need to set kthread_set_per_cpu().
> So it is already handled in the above code, the branch is unneeded.
>
Change this to something like fellowing.

	if (pool->flags & POOL_DISASSOCIATED)
		worker->flags |= WORKER_UNBOUND;
	else if (cpu_mask == pool->attrs->cpumask)
    	kthread_set_per_cpu(worker->task, pool->cpu);

I add this because we will set it = -1 on unbind_workers too.
static void unbind_workers(int cpu)
{
	...
			for_each_pool_worker(worker, pool) {
				kthread_set_per_cpu(worker->task, -1);
				WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task, wq_unbound_cpumask) < 0);
			}
    ...
}

>>
>>         list_add_tail(&worker->node, &pool->workers);
>>         worker->pool = pool;
>> @@ -1952,8 +1958,8 @@ 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->task->flags |= PF_NO_SETAFFINITY;
>>         /* successful, attach the worker to the pool */
>>         worker_attach_to_pool(worker, pool);
>>
>> --
>> 2.29.0
>>


-- 
BRs
Zhaohui Shi