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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Message-ID: <AT5PR8401MB1169D3DBEBC4F88D76BD1710AB650@AT5PR8401MB1169.NAMPRD84.PROD.OUTLOOK.COM>
Date:   Tue, 12 Feb 2019 03:37:22 +0000
From:   "Elliott, Robert (Persistent Memory)" <elliott@....com>
To:     Jens Axboe <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        Mikael Pettersson <mikpelinux@...il.com>
CC:     Linux SPARC Kernel Mailing List <sparclinux@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...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



> -----Original Message-----
> From: linux-kernel-owner@...r.kernel.org <linux-kernel-
> owner@...r.kernel.org> On Behalf Of Jens Axboe
> Sent: Monday, February 11, 2019 8:50 PM
> To: James Bottomley <James.Bottomley@...senPartnership.com>; 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
...

> > 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.

When blk-mq and scsi-mq were introduced to boost SAS SSDs over
1 million IOPS, this was purposely kept off, because it generated
a noticeable 3-12 us latency blip whenever the "fast_mix" code ran
out of bits. If Jens' patch changes the default to be enabled for
all scsi-mq users, that'll be a change for the newer legacy
scsi-mq users (except for users that don't trust the default and
always set the value in sysfs).