Re: [PATCH] SCSI: don't get target/host busy_count in scsi_mq_get_budget()

[Laurence Oberman <loberman@redhat.com>]

On Wed, 2017-11-08 at 10:57 -0700, Jens Axboe wrote:
> On 11/08/2017 09:41 AM, Bart Van Assche wrote:
> > On Tue, 2017-11-07 at 20:06 -0700, Jens Axboe wrote:
> > > At this point, I have no idea what Bart's setup looks like. Bart,
> > > it
> > > would be REALLY helpful if you could tell us how you are
> > > reproducing
> > > your hang. I don't know why this has to be dragged out.
> > 
> > Hello Jens,
> > 
> > It is a disappointment to me that you have allowed Ming to evaluate
> > other
> > approaches than reverting "blk-mq: don't handle TAG_SHARED in
> > restart". That
> > patch namely replaces an algorithm that is trusted by the community
> > with an
> > algorithm of which even Ming acknowledged that it is racy. A quote
> > from [1]:
> > "IO hang may be caused if all requests are completed just before
> > the current
> > SCSI device is added to shost->starved_list". I don't know of any
> > way to fix
> > that race other than serializing request submission and completion
> > by adding
> > locking around these actions, which is something we don't want.
> > Hence my
> > request to revert that patch.
> 
> I was reluctant to revert it, in case we could work out a better way
> of
> doing it. As I mentioned in the other replies, it's not exactly the
> prettiest or most efficient. However, since we currently don't have
> a good solution for the issue, I'm fine with reverting that patch.
> 
> > Regarding the test I run, here is a summary of what I mentioned in
> > previous
> > e-mails:
> > * I modified the SRP initiator such that the SCSI target queue
> > depth is
> >   reduced to one by setting starget->can_queue to 1 from inside
> >   scsi_host_template.target_alloc.
> > * With that modified SRP initiator I run the srp-test software as
> > follows
> >   until something breaks:
> >   while ./run_tests -f xfs -d -e deadline -r 60; do :; done
> 
> What kernel options are needed? Where do I download everything I
> need?
> 
> In other words, would it be possible to do a fuller guide for getting
> this setup and running?
> 
> I'll run my simple test case as well, since it's currently breaking
> basically everywhere.
> 
> > Today a system with at least one InfiniBand HCA is required to run
> > that test.
> > When I have the time I will post the SRP initiator and target
> > patches on the
> > linux-rdma mailing list that make it possible to run that test
> > against the
> > SoftRoCE driver (drivers/infiniband/sw/rxe). The only hardware
> > required to
> > use that driver is an Ethernet adapter.
> 
> OK, I guess I can't run it then... I'll have to rely on your testing.

Hello 

I agree with Bart in this case, we should revert this.
My test-bed is tied up and I have not been able to give it back to Ming
so he could follow up on Bart's last update.

Right now its safer to revert.

Thanks
Laurence
>