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

[Sumit Saxena <sumit.saxena@broadcom.com>]

On Thu, Feb 13, 2020 at 3:37 PM John Garry <john.garry@huawei.com> wrote:
>
> 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?
Updated patchset is not required. I will do performance run with this
change and update.

Thanks,
Sumit
>
> > Agreed, this should return correct HW queue index.
> >>
>
>
> Thanks,
> John