Re: linux-next: Tree for Aug 1

[Guenter Roeck]

On 08/01/2018 05:03 PM, James Bottomley wrote:
> On Thu, 2018-08-02 at 07:57 +0800, Ming Lei wrote:
> > On Thu, Aug 2, 2018 at 7:47 AM, Guenter Roeck <linux@xxxxxxxxxxxx>
> > wrote:
> > > On Wed, Aug 01, 2018 at 03:52:45PM -0700, James Bottomley wrote:
> > > > On Wed, 2018-08-01 at 15:48 -0700, Guenter Roeck wrote:
> > > > > On Wed, Aug 01, 2018 at 05:58:52PM +1000, Stephen Rothwell
> > > > > wrote:
> > > > > > Hi all,
> > > > > >
> > > > > > Changes since 20180731:
> > > > > >
> > > > > > The pci tree gained a conflict against the pci-current tree.
> > > > > >
> > > > > > The net-next tree gained a conflict against the bpf tree.
> > > > > >
> > > > > > The block tree lost its build failure.
> > > > > >
> > > > > > The staging tree still had its build failure due to an
> > > > > > interaction
> > > > > > with
> > > > > > the vfs tree for which I disabled CONFIG_EROFS_FS.
> > > > > >
> > > > > > The kspp tree lost its build failure.
> > > > > >
> > > > > > Non-merge commits (relative to Linus' tree): 10070
> > > > > >  Â9137 files changed, 417605 insertions(+), 179996 deletions(-
> > > > > > )
> > > > > >
> > > > > > -----------------------------------------------------------
> > > > > > ------
> > > > > > -----------
> > > > >
> > > > > The widespread kernel hang issues are still seen. I managed
> > > > > to bisect it after working around the transient build failures.
> > > > > Bisect log is attached below. Unfortunately, it doesn't help
> > > > > much.
> > > > > The culprit is reported as:
> > > > >
> > > > > 2d542828c5e9 Merge remote-tracking branch 'scsi/for-next'
> > > > >
> > > > > The preceding merge,
> > > > >
> > > > > 453f1d821165 Merge remote-tracking branch 'cgroup/for-next'
> > > > >
> > > > > checks out fine, as does the tip of scsi-next (commit
> > > > > 103c7b7e0184,
> > > > > "Merge branch 'misc' into for-next"). No idea how to proceed.
> > > >
> > > > This sounds like you may have a problem with this patch:
> > > >
> > > >  ÂÂÂÂcommit d5038a13eca72fb216c07eb717169092e92284f1
> > > >  ÂÂÂÂÂAuthor: Johannes Thumshirn <jthumshirn@xxxxxxx>
> > > >  ÂÂÂÂÂDate:ÂÂÂWed Jul 4 10:53:56 2018 +0200
> > > >
> > > >  ÂÂÂÂÂÂÂÂÂscsi: core: switch to scsi-mq by default
> > > >
> > > > To verify, boot with the additional kernel parameter
> > > >
> > > > scsi_mod.use_blk_mq=0
> > > >
> > > > Which will reverse the effect of the above patch.
> > >
> > > Yes, that fixes the problem.
> >
> > That may not the root cause, given this issue is only started to
> > see from next-20180731, but d5038a13eca7 (scsi: core: switch to
> > scsi-mq by default)
> > has been in -next for quite a while.
> >
> > Seems something new causes this issue.
>
> Read my other email about how to find this.
>
> https://marc.info/?l=linux-scsi&m=153316446223676
>
> Now that we've confirmed the issue, Gunter, could you attempt to bisect
> it as that email describes?

So, I am more and more baffled.

I ran another round of bisect, this time each test executing twice,
once with "scsi_mod.use_blk_mq=1" and once with "scsi_mod.use_blk_mq=0",
requiring both to pass. Bisect still points to the merge as culprit.

Ok, one step further: Actually _revert_ commit d5038a13eca72 before running
each test, meaning the default is use_blk_mq=0. Still run both tests.
Bisect _still_ points to the merge of scsi-next as culprit.

So, to me it looks like the problem is triggered by _something_ in
scsi-next, combined with _something_ in -next prior to the merge,
not specifically associated with use_blk_mq=[0|1] or d5038a13eca72,
but to a combination of some patch in scsi-next and some other patch.

I am running out of ideas. Any thoughts on how to track this down further ?

Guenter