Re: [RFC/RFT PATCH] scsi: pm8001: Expose HW queues for pm80xx hw

[John Garry]

On 11/01/2021 11:57, Jinpu Wang wrote:
> Hi John,
>
>
> On Tue, Jan 5, 2021 at 12:21 PM John Garry <john.garry@xxxxxxxxxx> wrote:
> > In commit 05c6c029a44d ("scsi: pm80xx: Increase number of supported
> > queues"), support for 80xx chip was improved by enabling multiple HW
> > queues.
> >
> > In this, like other SCSI MQ HBA drivers, the HW queues were not exposed
> > to upper layer, and instead the driver managed the queues internally.
> >
> > However, this management duplicates blk-mq code. In addition, the HW queue
> > management is sub-optimal for a system where the number of CPUs exceeds
> > the HW queues - this is because queues are selected in a round-robin
> > fashion, when it would be better to make adjacent CPUs submit on the same
> > queue. And finally, the affinity of the completion queue interrupts is not
> > set to mirror the cpu<->HQ queue mapping, which is suboptimal.
> >
> > As such, for when MSIX is supported, expose HW queues to upper layer. Flag
> > PCI_IRQ_AFFINITY is set for allocating the MSIX vectors to automatically
> > assign affinity for the completion queue interrupts.
> >
> > Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> >
> > ---
> > I sent as an RFC/RFT as I have no HW to test. In addition, since HW queue
> > #0 is used always for internal commands (like in send_task_abort()), if
> > all CPUs associated with HW queue #0 are offlined, the interrupt for that
> > queue will be shutdown, and no CPUs would be available to service any
> > internal commands completion. To solve that, we need [0] merged first and
> > switch over to use the new API. But we can still test performance in the
> > meantime.
> >
> > I assume someone else is making the change to use the request tag for IO
> > tag management.
> >
> > [0] https://lore.kernel.org/linux-scsi/47ba045e-a490-198b-1744-529f97192d3b@xxxxxxx/
> Thanks for the patch, maybe Viswas can help to test?

That's what I am hoping for :)

Thanks!

>