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:   Thu, 14 Feb 2019 19:35:22 +0100
From:   Mikael Pettersson <mikpelinux@...il.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     James Bottomley <James.Bottomley@...senpartnership.com>,
        Linux SPARC Kernel Mailing List 
        <sparclinux@...r.kernel.org>, linux-block@...r.kernel.org,
        linux-kernel@...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 Sun, Feb 10, 2019 at 5:05 PM Jens Axboe <axboe@...nel.dk> wrote:
>
> 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@...senpartnership.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;
>  }

This patch eliminates my 5 minute boot-up delay problem.

/Mikael