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

From: Jens Axboe
Date: Mon Feb 11 2019 - 21:50:35 EST

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.

Jens Axboe