Re: [PATCH] blk-mq: only run mapped hw queues in blk_mq_run_hw_queues()

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 04/06/2018 11:23 AM, Ming Lei wrote:
> On Fri, Apr 06, 2018 at 10:51:28AM +0200, Christian Borntraeger wrote:
>>
>>
>> On 04/06/2018 10:41 AM, Ming Lei wrote:
>>> On Thu, Apr 05, 2018 at 07:39:56PM +0200, Christian Borntraeger wrote:
>>>>
>>>>
>>>> On 04/05/2018 06:11 PM, Ming Lei wrote:
>>>>>>
>>>>>> Could you please apply the following patch and provide the dmesg boot log?
>>>>>
>>>>> And please post out the 'lscpu' log together from the test machine too.
>>>>
>>>> attached.
>>>>
>>>> As I said before this seems to go way with CONFIG_NR_CPUS=64 or smaller.
>>>> We have 282 nr_cpu_ids here (max 141CPUs on that z13 with SMT2) but only 8 Cores
>>>> == 16 threads.
>>>
>>> OK, thanks!
>>>
>>> The most weird thing is that hctx->next_cpu is computed as 512 since
>>> nr_cpu_id is 282, and hctx->next_cpu should have pointed to one of
>>> possible CPU.
>>>
>>> Looks like it is a s390 specific issue, since I can setup one queue
>>> which has same mapping with yours:
>>>
>>> 	- nr_cpu_id is 282
>>> 	- CPU 0~15 is online
>>> 	- 64 queues null_blk
>>> 	- still run all hw queues in .complete handler
>>>
>>> But can't reproduce this issue at all.
>>>
>>> So please test the following patch, which may tell us why hctx->next_cpu
>>> is computed wrong:
>>
>> I see things like
>>
>> [    8.196907] wrong next_cpu 512, blk_mq_map_swqueue, first_and
>> [    8.196910] wrong next_cpu 512, blk_mq_map_swqueue, first_and
>> [    8.196912] wrong next_cpu 512, blk_mq_map_swqueue, first_and
>> [    8.196913] wrong next_cpu 512, blk_mq_map_swqueue, first_and
>> [    8.196914] wrong next_cpu 512, blk_mq_map_swqueue, first_and
>> [    8.196915] wrong next_cpu 512, blk_mq_map_swqueue, first_and
>> [    8.196916] wrong next_cpu 512, blk_mq_map_swqueue, first_and
>> [    8.196916] wrong next_cpu 512, blk_mq_map_swqueue, first_and
>> [    8.196917] wrong next_cpu 512, blk_mq_map_swqueue, first_and
>> [    8.196918] wrong next_cpu 512, blk_mq_map_swqueue, first_and
>>
>> which is exactly what happens if the find and and operation fails (returns size of bitmap).
> 
> Given both 'cpu_online_mask' and 'hctx->cpumask' are shown as correct
> in your previous debug log, it means the following function returns
> totally wrong result on S390.
> 
> 	cpumask_first_and(hctx->cpumask, cpu_online_mask);
> 
> The debugfs log shows that each hctx->cpumask includes one online
> CPU(0~15).
> 
> So looks it isn't one issue in block MQ core.

So I checked further and printed the mask I think I can ignore the next_and cases. It is totally
valid to get 512 here (as we might start with an offset that is already the last cpu and we need
to wrap with first_and)).

So the first_and and the first cases are really the interesting one. And I think the code is perfectly
right, there is no bit after and for these cases:


[    3.220021] wrong next_cpu 512, blk_mq_map_swqueue, first_and
[    3.220023] 1: 0000000000010000 0000000000010000 0000000000010000 0000000000010000 0000000000010000 0000000000000000 0000000000000000 0000000000000000
[    3.220025] 2: 000000000000ffff 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[    3.220027] wrong next_cpu 512, blk_mq_map_swqueue, first_and
[    3.220028] 1: 0000000000020000 0000000000020000 0000000000020000 0000000000020000 0000000000020000 0000000000000000 0000000000000000 0000000000000000
[    3.220030] wrong next_cpu 512, blk_mq_map_swqueue, first_and
[    3.220032] 1: 0000000000010000 0000000000010000 0000000000010000 0000000000010000 0000000000010000 0000000000000000 0000000000000000 0000000000000000
[    3.220033] 2: 000000000000ffff 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[    3.220035] wrong next_cpu 512, blk_mq_map_swqueue, first_and
[    3.220036] 2: 000000000000ffff 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[    3.220037] wrong next_cpu 512, blk_mq_map_swqueue, first_and
[    3.220039] 1: 0000000000040000 0000000000040000 0000000000040000 0000000000040000 0000000000040000 0000000000000000 0000000000000000 0000000000000000
[    3.220040] 2: 000000000000ffff 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[    3.220042] wrong next_cpu 512, blk_mq_map_swqueue, first_and
[    3.220062] 1: 0000000000020000 0000000000020000 0000000000020000 0000000000020000 0000000000020000 0000000000000000 0000000000000000 0000000000000000
[    3.220063] 2: 000000000000ffff 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000
[    3.220064] wrong next_cpu 512, blk_mq_map_swqueue, first_and
[    3.220066] 1: 0000000000080000 0000000000080000 0000000000080000 0000000000080000 0000000000080000 0000000000000000 0000000000000000 0000000000000000
[    3.220067] 2: 000000000000ffff 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000 0000000000000000