Re: [5.0-rc5 regression] "scsi: kill off the legacy IO path" causes 5 minute delay during boot on Sun Blade 2500

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2019-02-11 at 19:50 -0700, Jens Axboe wrote:
> On 2/11/19 7:13 PM, James Bottomley wrote:
> > On Mon, 2019-02-11 at 09:31 -0700, Jens Axboe wrote:
> > > On 2/11/19 9:28 AM, James Bottomley wrote:
> > > > On Mon, 2019-02-11 at 08:46 -0700, Jens Axboe wrote:
> > > > > On 2/11/19 8:42 AM, James Bottomley wrote:
> > > > > > On Mon, 2019-02-11 at 08:28 -0700, Jens Axboe wrote:
> > > > > > > On 2/11/19 8:25 AM, James Bottomley wrote:
> > > > > > > > On Sun, 2019-02-10 at 09:35 -0700, Jens Axboe wrote:
> > > > > > > > > On 2/10/19 9:25 AM, James Bottomley wrote:
> > > > 
> > > > [...]
> > > > > > > > > > That check wasn't changed by the code removal.
> > > > > > > > > 
> > > > > > > > > As I said above, for sd. This isn't true for non-
> > > > > > > > > disks.
> > > > > > > > 
> > > > > > > > Yes, but the behaviour above doesn't change across a
> > > > > > > > switch
> > > > > > > > to MQ, so I don't quite understand how it bisects back
> > > > > > > > to
> > > > > > > > that change.  If we're not gathering entropy for the
> > > > > > > > device
> > > > > > > > now, we wouldn't have been before the switch, so the
> > > > > > > > entropy characteristics shouldn't have changed.
> > > > > > > 
> > > > > > > But it does, as I also wrote in that first email. The
> > > > > > > legacy
> > > > > > > queue flags had QUEUE_FLAG_ADD_RANDOM set by default, the
> > > > > > > MQ
> > > > > > > ones do not. Hence any non-sd device would previously
> > > > > > > ALWAYS
> > > > > > > have ADD_RANDOM set, now none of them do. Also see the
> > > > > > > patch
> > > > > > > I sent.
> > > > > > 
> > > > > > So your theory is that the disk in question never gets to
> > > > > > the
> > > > > > rotational check?  because the check will clear the flag if
> > > > > > it's non-rotational and set it if it's not, so the default
> > > > > > state of the flag shouldn't matter.
> > > > > 
> > > > > No, my point is about non-disks, devices that aren't driven
> > > > > by
> > > > > sd. The behavior for sd hasn't changed, as it sets/clears it
> > > > > unconditionally. 
> > > > 
> > > > I agree, but I don't think any of them were significant entropy
> > > > contributors before: things like nvme have always been outside
> > > > of
> > > > this and sr and st don't really contribute much to the seek
> > > > load
> > > > during boot because they're probed but not used by the boot
> > > > sequence, so I can't see how they would cause this
> > > > behaviour.  I
> > > > suppose it could be target probing, but even that seems
> > > > unlikely
> > > > because it should be dwarfed by the number of root disk reads
> > > > during boot.
> > > > 
> > > > For the rng to take an additional 5 minutes to initialize, we
> > > > must
> > > > have lost a significant entropy source somewhere.
> > > 
> > > I agree it's not a significant amount of entropy, but even just
> > > one
> > > bit could mean a long stall if that put us over the edge of just
> > > not
> > > having enough for whatever is blocking on /dev/random. Mikael's
> > > boot
> > > did have a CDROM, it's not impossible that the handful of
> > > commands we
> > > end up doing to that device would have contributed enough entropy
> > > to
> > > get the boot done without stalling for minutes.
> > > 
> > > One way to know for sure, and that's if Mikael tests the patch.
> > 
> > I think I've got the root cause.  I have one system in my test bed
> > exhibiting this behaviour.  It turns out the disk in it has no
> > characteristics VPD page.  The 0xB1 VPD was a SBC-3 addition, so
> > that's
> > not surprising.  However, the characteristics check bails before
> > setting the flags, so it takes the default flag which has flipped.
> > 
> > We can either fix this by setting the QUEUE_FLAG_ADD_RANDOM if
> > there's
> > no 0xB1 page or by setting the default as Jens proposed.
> 
> I'd recommend just doing my patch, since that'll be the same behavior
> that SCSI had before.

I've got the history now, it's this patch

Author: Xuewei Zhang <xueweiz@google.com>
Date:   Thu Sep 6 13:37:19 2018 -0700

    scsi: sd: Contribute to randomness when running rotational device

It added the else branch to the if (rot == 1).  It's the position of
that else branch which is wrong because not all disks have a SBC-3
characteristics VPD page, so they're the ones under MQ which stop
contributing entropy.  Whichever patch we go with will need a fixes:
for this.

James