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

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Date:   Mon, 11 Feb 2019 18:13:18 -0800
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Jens Axboe <axboe@...nel.dk>,
        Mikael Pettersson <mikpelinux@...il.com>
Cc:     Linux SPARC Kernel Mailing List <sparclinux@...r.kernel.org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [5.0-rc5 regression] "scsi: kill off the legacy IO path" causes
 5 minute delay during boot on Sun Blade 2500

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.

James

---

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index d0a980915801..1f3a1474042e 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -2961,6 +2961,10 @@ static void sd_read_block_characteristics(struct scsi_disk *sdkp)
 
 	buffer = kmalloc(vpd_len, GFP_KERNEL);
 
+	/* set to rotational in case no device characteristic page exists */
+	blk_queue_flag_clear(QUEUE_FLAG_NONROT, q);
+	blk_queue_flag_set(QUEUE_FLAG_ADD_RANDOM, q);
+
 	if (!buffer ||
 	    /* Block Device Characteristics VPD */
 	    scsi_get_vpd_page(sdkp->device, 0xb1, buffer, vpd_len))
@@ -2971,9 +2975,6 @@ static void sd_read_block_characteristics(struct scsi_disk *sdkp)
 	if (rot == 1) {
 		blk_queue_flag_set(QUEUE_FLAG_NONROT, q);
 		blk_queue_flag_clear(QUEUE_FLAG_ADD_RANDOM, q);
-	} else {
-		blk_queue_flag_clear(QUEUE_FLAG_NONROT, q);
-		blk_queue_flag_set(QUEUE_FLAG_ADD_RANDOM, q);
 	}
 
 	if (sdkp->device->type == TYPE_ZBC) {