Re: [PATCH 2/2] blk-mq: simplify queue mapping & schedule with each possisble CPU

["jianchao.wang" <jianchao.w.wang@oracle.com>]

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Hi ming

Thanks for your kindly response and directive.

On 01/16/2018 10:31 PM, jianchao.wang wrote:
> Hi minglei
> 
> On 01/16/2018 08:10 PM, Ming Lei wrote:
>>>> -		next_cpu = cpumask_next(hctx->next_cpu, hctx->cpumask);
>>>> +		next_cpu = cpumask_next_and(hctx->next_cpu, hctx->cpumask,
>>>> +				cpu_online_mask);
>>>>  		if (next_cpu >= nr_cpu_ids)
>>>> -			next_cpu = cpumask_first(hctx->cpumask);
>>>> +			next_cpu = cpumask_first_and(hctx->cpumask,cpu_online_mask);
>>> the next_cpu here could be >= nr_cpu_ids when the none of on hctx->cpumask is online.
>> That supposes not happen because storage device(blk-mq hw queue) is
>> generally C/S model, that means the queue becomes only active when
>> there is online CPU mapped to it.
>>
>> But it won't be true for non-block-IO queue, such as HPSA's queues[1], and
>> network controller RX queues.
>>
>> [1] https://urldefense.proofpoint.com/v2/url?u=https-3A__marc.info_-3Fl-3Dlinux-2Dkernel-26m-3D151601867018444-26w-3D2&d=DwIBAg&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=7WdAxUBeiTUTCy8v-7zXyr4qk7sx26ATvfo6QSTvZyQ&m=tCZdQH6JUW1dkNCN92ycoUoKfDU_qWj-7EsUoYpOeJ0&s=vgHC9sbjYQb7mtY9MUJzbVXyVEyjoNJPWEx4_rfrHxU&e=
>>
>> One thing I am still not sure(but generic irq affinity supposes to deal with
>> well) is that the CPU may become offline after the IO is just submitted,
>> then where the IRQ controller delivers the interrupt of this hw queue
>> to?
>>
>>> This could be reproduced on NVMe with a patch that could hold some rqs on ctx->rq_list,
>>> meanwhile a script online and offline the cpus. Then a panic occurred in __queue_work().
>> That shouldn't happen, when CPU offline happens the rqs in ctx->rq_list
>> are dispatched directly, please see blk_mq_hctx_notify_dead().
> 
> Yes, I know. The  blk_mq_hctx_notify_dead will be invoked after the cpu has been set offlined.
> Please refer to the following diagram.
> 
> CPU A                      CPU T
>                  kick  
>   _cpu_down()     ->       cpuhp_thread_fun (cpuhpT kthread)
>                                AP_ACTIVE           (clear cpu_active_mask)
>                                  |
>                                  v
>                                AP_WORKQUEUE_ONLINE (unbind workers)
>                                  |
>                                  v
>                                TEARDOWN_CPU        (stop_machine)
>                                     ,                   | execute
>                                      \_ _ _ _ _ _       v
>                                         preempt  V  take_cpu_down ( migration kthread)
>                                                     set_cpu_online(smp_processor_id(), false) (__cpu_disable)  ------> Here !!!
>                                                     TEARDOWN_CPU
>                                                         |
>              cpuhpT kthead is    |                      v
>              migrated away       ,                    AP_SCHED_STARTING (migrate_tasks)
>                  _ _ _ _ _ _ _ _/                       |
>                 V                                       v
>               CPU X                                   AP_OFFLINE
>                                                         
>                                                         |
>                                                         ,
>                                              _ _ _ _ _ /
>                                             V
>                                       do_idle (idle task)
>  <_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ cpuhp_report_idle_dead
>                          complete st->done_down
>            __cpu_die (cpuhpT kthread, teardown_cpu) 
> 
>  AP_OFFLINE
>    |
>    v
>  BRINGUP_CPU
>    |
>    v
>  BLK_MQ_DEAD    -------> Here !!!
>    |
>    v
>  OFFLINE
> 
> The cpu has been cleared in cpu_online_mask when blk_mq_hctx_notify_dead is invoked.
> If the device is NVMe which only has one cpu mapped on the hctx, 
> cpumask_first_and(hctx->cpumask,cpu_online_mask) will return a bad value.
> 
> I even got a backtrace showed that the panic occurred in blk_mq_hctx_notify_dead -> kblocked_schedule_delayed_work_on -> __queue_work.
> Kdump doesn't work well on my machine, so I cannot share the backtrace here, that's really sad.
> I even added BUG_ON as following, it could be triggered.
>>>>>
> 	if (next_cpu >= nr_cpu_ids)
> 		next_cpu = cpumask_first_and(hctx->cpumask,cpu_online_mask);
> 	BUG_ON(next_cpu >= nr_cpu_ids);
>>>>>
> 
I find two pictures taken when I did the test. Share them here. 
Please refer.
The PANIC.jpg is the panic scenario.
The BUG_ON.jpg is the scenario when the BUG_ON added as above was triggered. 

Thanks
Jianchao

>>
>>> maybe cpu_possible_mask here, the workers in the pool of the offlined cpu has been unbound.
>>> It should be ok to queue on them.
> 

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