Re: [PATCH 09/11] megaraid_sas: switch fusion adapters to MQ

[John Garry <john.garry@huawei.com>]

On 17/01/2020 11:18, Sumit Saxena wrote:
>>>> or doing a performance test here.
>>> Hi Hannes,
>>>

Hi Sumit,

>>> Sorry for the delay in replying, I observed a few issues with this patchset:
>>>
>>> 1. "blk_mq_unique_tag_to_hwq(tag)" does not return MSI-x vector to
>>> which IO submitter CPU is affined with. Due to this IO submission and
>>> completion CPUs are different which causes performance drop for low
>>> latency workloads.
>> Hi Sumit,
>>
>> So the new code has:
>>
>> megasas_build_ldio_fusion()
>> {
>>
>> cmd->request_desc->SCSIIO.MSIxIndex =
>> blk_mq_unique_tag_to_hwq(tag);
>>
>> }
>>
>> So the value here is hw queue index from blk-mq point of view, and not
>> megaraid_sas msix index, as you alluded to.
> Yes John, value filled in "cmd->request_desc->SCSIIO.MSIxIndex" is HW
> queue index.
> 
>> So we get 80 msix, 8 are reserved for low_latency_index_start (that's
>> how it seems to me), and we report other 72 as #hw queues = 72 to SCSI
>> midlayer.
>>
>> So I think that this should be:
>>
>> cmd->request_desc->SCSIIO.MSIxIndex =
>> blk_mq_unique_tag_to_hwq(tag) + low_latency_index_start;

Can you possibly test performance again with this local change, or would 
you rather an updated patchset be sent?

> Agreed, this should return correct HW queue index.
>>


Thanks,
John