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

Jens Axboe
2019-02-10T16:05:39Z

On 2/10/19 8:44 AM, James Bottomley wrote:
> On Sun, 2019-02-10 at 10:17 +0100, Mikael Pettersson wrote:
>> On Sat, Feb 9, 2019 at 7:19 PM James Bottomley
>> <James.Bottomley@hansenpartnership.com> wrote:
> [...]
>>> I think the reason for this is that the block mq path doesn't feed
>>> the kernel entropy pool correctly, hence the need to install an
>>> entropy gatherer for systems that don't have other good random
>>> number sources.
>>
>> That does sound plausible, I admit I didn't even consider the
>> possibility that the old block I/O path also was an entropy source.
>
> In theory, the new one should be as well since the rotational entropy
> collector is on the SCSI completion path. I'd seen the same problem
> but had assumed it was something someone had done to our internal
> entropy pool and thus hadn't bisected it.

The difference is that the old stack included ADD_RANDOM by default,
so this check:

if (blk_queue_add_random(q))
add_disk_randomness(req->rq_disk);

in scsi_end_request() would be true, and we'd add the randomness. For
sd, it seems to set it just fine for non-rotational drives. Could this
be because other devices don't? Maybe the below makes a difference.

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 6d65ac584eba..60e029911755 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1881,6 +1881,7 @@ struct request_queue *scsi_mq_alloc_queue(struct scsi_device *sdev)
sdev->request_queue->queuedata = sdev;
__scsi_init_queue(sdev->host, sdev->request_queue);
blk_queue_flag_set(QUEUE_FLAG_SCSI_PASSTHROUGH, sdev->request_queue);
+ blk_queue_flag_set(QUEUE_FLAG_ADD_RANDOM, sdev->request_queue);
return sdev->request_queue;
}

--
Jens Axboe