Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [5.0-rc5 regression] "scsi: kill off the legacy IO path" causes 5
 minute delay during boot on Sun Blade 2500
To:     James Bottomley <James.Bottomley@HansenPartnership.com>,
        Mikael Pettersson <mikpelinux@gmail.com>
Cc:     Linux SPARC Kernel Mailing List <sparclinux@vger.kernel.org>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-scsi <linux-scsi@vger.kernel.org>
References: <CAM43=SO6bdDAEULEPArfgdbjLAYDo9OiPBa8gL4fsSg7Dky=gA@mail.gmail.com>
 <1549736341.2971.7.camel@HansenPartnership.com>
 <CAM43=SPUMzOLRmkoT+DQSpzu2DYRTyOhU6o8KRUU_Q902o2e1Q@mail.gmail.com>
 <1549813472.4142.3.camel@HansenPartnership.com>
 <3380ed8e-ae02-96f2-142b-7cce09459df8@kernel.dk>
 <1549815924.4142.8.camel@HansenPartnership.com>
 <0e6e5d67-d305-dd00-2e42-e2299166c8b2@kernel.dk>
 <1549898730.2831.6.camel@HansenPartnership.com>
 <44bb4374-0b7c-733b-a53e-92d2f03f2f49@kernel.dk>
 <1549899773.2831.12.camel@HansenPartnership.com>
 <1a00da0e-cb8e-30ea-8d17-120f97242b2f@kernel.dk>
 <1549902521.2831.23.camel@HansenPartnership.com>
 <fce66227-ff41-3a4e-4c6a-f4c2ca492b95@kernel.dk>
 <1549937598.2857.8.camel@HansenPartnership.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <fc547d40-9a42-a823-b62f-460c3244fe7d@kernel.dk>
Date:   Mon, 11 Feb 2019 19:50:28 -0700
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.4.0
MIME-Version: 1.0
In-Reply-To: <1549937598.2857.8.camel@HansenPartnership.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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