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

Christian Borntraeger
2018-04-06T15:11:53Z

On 04/06/2018 04:58 PM, Ming Lei wrote:
> On Fri, Apr 06, 2018 at 04:26:49PM +0200, Christian Borntraeger wrote:
>>
>>
>> On 04/06/2018 03:41 PM, Ming Lei wrote:
>>> On Fri, Apr 06, 2018 at 12:19:19PM +0200, Christian Borntraeger wrote:
>>>>
>>>>
>>>> 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).
>>>>
>>>> Really? the last log (with the latest patch applied shows a lot of contexts
>>>> that do not have CPUs in 0-15:
>>>>
>>>> e.g.
>>>> [ 4.049828] dump CPUs mapped to this hctx:
>>>> [ 4.049829] 18
>>>> [ 4.049829] 82
>>>> [ 4.049830] 146
>>>> [ 4.049830] 210
>>>> [ 4.049831] 274
>>>
>>> That won't be an issue, since no IO can be submitted from these offline
>>> CPUs, then these hctx shouldn't have been run at all.
>>>
>>> But hctx->next_cpu can be set as 512 for these inactive hctx in
>>> blk_mq_map_swqueue(), then please test the attached patch, and if
>>> hctx->next_cpu is still set as 512, something is still wrong.
>>
>>
>> WIth this patch I no longer see the "run queue from wrong CPU x, hctx active" messages.
>> your debug code still triggers, though.
>>
>> wrong next_cpu 512, blk_mq_hctx_next_cpu, first_and
>> wrong next_cpu 512, blk_mq_hctx_next_cpu, next_and
>>
>> If we would remove the debug code then dmesg would be clean it seems.
>
> That is still a bit strange, since for any inactive hctx(without online
> CPU mapped), blk_mq_run_hw_queue() will check blk_mq_hctx_has_pending()

I think for next_and it is reasonable to see this, as the next_and will return
512 after we have used the last one. In fact the code does call first_and in
that case for a reason, no?

> first. And there shouldn't be any pending IO for all inactive hctx
> in your case, so looks blk_mq_hctx_next_cpu() shouldn't be called for
> inactive hctx.
>
> I will prepare one patchset and post out soon, and hope all these issues
> can be covered.
>
> Thanks,
> Ming
>