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Date:	Fri, 19 Sep 2014 13:13:11 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Christoph Hellwig <hch@....de>, Tejun Heo <tj@...nel.org>
CC:	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: boot stall regression due to blk-mq: use percpu_ref for mq usage
 count

On 09/19/2014 05:38 AM, Christoph Hellwig wrote:
> Hi Jens, hi Tejun,
> 
> I've seen multi-second boot stalls in one of my KVM setups during
> the initial scsi scan:
> 
> [    0.949892] scsi host0: Virtio SCSI HBA
> [    1.007864] scsi 0:0:0:0: Direct-Access     QEMU     QEMU HARDDISK    1.1. PQ: 0 ANSI: 5
> [    1.021299] scsi 0:0:1:0: Direct-Access     QEMU     QEMU HARDDISK    1.1. PQ: 0 ANSI: 5
> [    1.520356] tsc: Refined TSC clocksource calibration: 2491.910 MHz
> 
> <stall>
> 
> [   16.186549] sd 0:0:0:0: Attached scsi generic sg0 type 0
> [   16.190478] sd 0:0:1:0: Attached scsi generic sg1 type 0
> [   16.194099] osd: LOADED open-osd 0.2.1
> [   16.203202] sd 0:0:0:0: [sda] 31457280 512-byte logical blocks: (16.1 GB/15.0 GiB)
> [   16.208478] sd 0:0:0:0: [sda] Write Protect is off
> [   16.211439] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
> [   16.218771] sd 0:0:1:0: [sdb] 31457280 512-byte logical blocks: (16.1 GB/15.0 GiB)
> [   16.223264] sd 0:0:1:0: [sdb] Write Protect is off
> [   16.225682] sd 0:0:1:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
> 
> 
> 
> I've tracked this down to "blk-mq: use percpu_ref for mq usage count" in
> a rather painful way as that one introduced enough other regressions
> to mess up bisect.
> 
> If I revert the following commits:
> 
> dd840087086f3b93ac20f7472b4fca59aff7b79f
> cddd5d17642cc6881352732693c2ae6930e9ce65
> add703fda981b9719d37f371498b9f129acbd997
> 
> which are the above mentioned commit and two fixes to it the problem goes
> away.

Thanks for bisecting this, I ran into something that I think is the same
issue about a week ago. Tejun, any ideas before I dig into this one?

-- 
Jens Axboe

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