Re: [PATCH V3 8/8] scsi: megaraid: improve scsi_mq performance via .host_tagset

[Ming Lei <ming.lei@redhat.com>]

On Thu, Mar 08, 2018 at 03:34:31PM +0530, Kashyap Desai wrote:
> > -----Original Message-----
> > From: Ming Lei [mailto:ming.lei@redhat.com]
> > Sent: Thursday, March 8, 2018 6:46 AM
> > To: Kashyap Desai
> > Cc: Jens Axboe; linux-block@vger.kernel.org; Christoph Hellwig; Mike
> Snitzer;
> > linux-scsi@vger.kernel.org; Hannes Reinecke; Arun Easi; Omar Sandoval;
> > Martin K . Petersen; James Bottomley; Christoph Hellwig; Don Brace;
> Peter
> > Rivera; Laurence Oberman
> > Subject: Re: [PATCH V3 8/8] scsi: megaraid: improve scsi_mq performance
> via
> > .host_tagset
> >
> > On Wed, Mar 07, 2018 at 10:58:34PM +0530, Kashyap Desai wrote:
> > > > >
> > > > > Also one observation using V3 series patch. I am seeing below
> > > > > Affinity mapping whereas I have only 72 logical CPUs.  It means we
> > > > > are really not going to use all reply queues.
> > > > > e.a If I bind fio jobs on CPU 18-20, I am seeing only one reply
> > > > > queue is used and that may lead to performance drop as well.
> > > >
> > > > If the mapping is in such shape, I guess it should be quite
> > > > difficult to
> > > figure out
> > > > one perfect way to solve this situation because one reply queue has
> > > > to
> > > handle
> > > > IOs submitted from 4~5 CPUs at average.
> > >
> > > 4.15.0-rc1 kernel has below mapping - I am not sure which commit id in
> "
> > > linux_4.16-rc-host-tags-v3.2" is changing the mapping of IRQ to CPU.
> > > It
> >
> > I guess the mapping you posted is read from /proc/irq/126/smp_affinity.
> >
> > If yes, no any patch in linux_4.16-rc-host-tags-v3.2 should change IRQ
> affinity
> > code, which is done in irq_create_affinity_masks(), as you saw, no any
> patch
> > in linux_4.16-rc-host-tags-v3.2 touches that code.
> >
> > Could you simply apply the patches in linux_4.16-rc-host-tags-v3.2
> against
> > 4.15-rc1 kernel and see any difference?
> >
> > > will be really good if we can fall back to below mapping once again.
> > > Current repo linux_4.16-rc-host-tags-v3.2 is giving lots of random
> > > mapping of CPU - MSIx. And that will be problematic in performance
> run.
> > >
> > > As I posted earlier, latest repo will only allow us to use *18* reply
> >
> > Looks not see this report before, could you share us how you conclude
> that?
> > The only patch changing reply queue is the following one:
> >
> > 	https://marc.info/?l=linux-block&m=151972611911593&w=2
> >
> > But not see any issue in this patch yet, can you recover to 72 reply
> queues
> > after reverting the patch in above link?
> Ming -
> 
> While testing, my system went bad. I debug further and understood that
> affinity mapping was changed due to below commit -
> 84676c1f21e8ff54befe985f4f14dc1edc10046b
> 
> [PATCH] genirq/affinity: assign vectors to all possible CPUs
> 
> Because of above change, we end up using very less reply queue. Many reply
> queues on my setup was mapped to offline/not-available CPUs. This may be
> primary contributing to odd performance impact and it may not be truly due
> to V3/V4 patch series.

Seems a good news, :-)

> 
> I am planning to check your V3 and V4 series after removing above commit
> ID (for performance impact.).

You can run your test on a server in which all CPUs are kept as online
for avoiding this issue.

Or you can apply the following patchset for avoiding this issue:

	https://marc.info/?l=linux-block&m=152050646332092&w=2

> 
> It is good if we spread possible CPUs (instead of online cpus) to all irq
> vectors  considering -  We should have at least *one* online CPU mapped to
> the vector.

Right, that is exactly what the above patchset does.

Thanks,
Ming