Re: [PATCH RFC v7 10/12] megaraid_sas: switch fusion adapters to MQ

Ming Lei
2020-07-21T01:13:48Z

On Mon, Jul 20, 2020 at 12:53:55PM +0530, Kashyap Desai wrote:
> > > > I also noticed nr_hw_queues are now exposed in sysfs -
> > > >
> > > >
> > /sys/devices/pci0000:85/0000:85:00.0/0000:86:00.0/0000:87:04.0/0000:8b
> > > >
> > >
> > :00.0/0000:8c:00.0/0000:8d:00.0/host14/scsi_host/host14/nr_hw_queues:1
> > > > 28
> > > > .
> > >
> > > That's on my v8 wip branch, so I guess you're picking it up from there.
> >
> > John - I did more testing on v8 wip branch. CPU hotplug is working as
> > expected, but I still see some performance issue on Logical Volumes.
> >
> > I created 8 Drives Raid-0 VD on MR controller and below is performance
> > impact of this RFC. Looks like contention is on single <sdev>.
> >
> > I used command - "numactl -N 1 fio 1vd.fio --iodepth=128 --bs=4k --
> > rw=randread --cpus_allowed_policy=split --ioscheduler=none --
> > group_reporting --runtime=200 --numjobs=1"
> > IOPS without RFC = 300K IOPS with RFC = 230K.
> >
> > Perf top (shared host tag. IOPS = 230K)
> >
> > 13.98% [kernel] [k] sbitmap_any_bit_set
> > 6.43% [kernel] [k] blk_mq_run_hw_queue
>
> blk_mq_run_hw_queue function take more CPU which is called from "
> scsi_end_request"

The problem could be that nr_hw_queues is increased a lot so that
sample on blk_mq_run_hw_queue() can be observed now.

> It looks like " blk_mq_hctx_has_pending" handles only elevator (scheduler)
> case. If queue has ioscheduler=none, we can skip. I case of scheduler=none,
> IO will be pushed to hardware queue and it by pass software queue.
> Based on above understanding, I added below patch and I can see performance
> scale back to expectation.
>
> Ming mentioned that - we cannot remove blk_mq_run_hw_queues() from IO
> completion path otherwise we may see IO hang. So I have just modified
> completion path assuming it is only required for IO scheduler case.
> https://www.spinics.net/lists/linux-block/msg55049.html
>
> Please review and let me know if this is good or we have to address with
> proper fix.
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 1be7ac5a4040..b6a5b41b7fc2 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1559,6 +1559,9 @@ void blk_mq_run_hw_queues(struct request_queue *q,
> bool async)
> struct blk_mq_hw_ctx *hctx;
> int i;
>
> + if (!q->elevator)
> + return;
> +

This way shouldn't be correct, blk_mq_run_hw_queues() is still needed for
none because request may not be dispatched successfully by direct issue.

Thanks,
Ming