RE: [RFC] bypass scheduler for no sched is set

[Kashyap Desai <kashyap.desai@broadcom.com>]

Ming, Thanks for checking this request and sending appropriate changes.
This RFC changes are settled down with proper block layer changes (below
patch) sent by Ming.

https://www.spinics.net/lists/linux-block/msg28279.html

Kashyap

> -----Original Message-----
> From: Ming Lei [mailto:ming.lei@redhat.com]
> Sent: Wednesday, July 4, 2018 2:38 PM
> To: Kashyap Desai
> Cc: linux-block@vger.kernel.org; Jens Axboe; Omar Sandoval; Christoph
> Hellwig; Hannes Reinecke; linux-scsi; Laurence Oberman
> Subject: Re: [RFC] bypass scheduler for no sched is set
>
> On Wed, Jul 04, 2018 at 01:29:50PM +0530, Kashyap Desai wrote:
> > Hi,
> >
> > Ming Lei posted below patch series and performance improved for
> > megaraid_sas driver. I used the same kernel base and figure out some
more
> > possible performance improvement in block layer. This RFC improves
> > performance as well as CPU utilization. If this patch fits the design
> > aspect of the blk-mq and scsi-mq, I can convert it into PATCH and
submit
> > the same/modified version.
> >
> > https://marc.info/?l=linux-block&m=153062994403732&w=2
> >
> > Description of change -
> >
> > Do not insert request into software queue if BLK_MQ_F_NO_SCHED is set.
> > Submit request from blk_mq_make_request to low level driver directly
as
> > depicted through below function call.
> >
> > blk_mq_try_issue_directly
> >
> >
> > __blk_mq_try_issue_directly
> >
> >                  scsi_queue_rq
>
> Hi Kashyap,
>
> When I sent you the patches[1] which include 'global tags' support,
MegaRAID
> is converted to per-node queues and becomes real MQ(q->nr_hw_queues >
> 1), IO
> should have been issued direclty, please see the branch of '(plug &&
> !blk_queue_nomerges(q))' and '(q->nr_hw_queues > 1 && is_sync)' in
> blk_mq_make_request().
>
> But looks not see any improvement from your test result, even though
huge
> improvement can be observed on null_blk/scsi_debug. Maybe somewhere is
> wrong,
> and days ago I talked with Laurence about doing this kind of test again.
>
> I will double check the 'global tags' patches, meantime could you or
> Laurence help to check if global tags[2] works in expected way if
> you'd like to?
>
> [1] https://github.com/ming1/linux/commits/v4.16-rc-host-tags-v5
> [2] https://github.com/ming1/linux/commits/v4.18-rc-host-tags-v8
> >
> > Low level driver attached to scsi.mq can set BLK_MQ_F_NO_SCHED,  If
they
> do
> > not want benefit from io scheduler (e.a in case of SSDs connected to
IT/MR
> > controller). In case of HDD drives connected to HBA, driver can avoid
> > setting BLK_MQ_F_NO_SCHED so that default elevator is set to mq-
> deadline.
>
> That might be one way for improving megaraid_sas.
>
> However, even for modern high performance device, such as NVMe, turns
out
> IO scheduler(such as kyber) may play a big role for improving latency or
> throughput.
>
>
> Thanks,
> Ming