linux-kernel - Re: boot stall regression due to blk-mq: use percpu

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Message-ID: <20140923060346.GG11740@mtj.dyndns.org>
Date:	Tue, 23 Sep 2014 02:03:46 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Hellwig <hch@....de>
Cc:	Jens Axboe <axboe@...nel.dk>, 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 Tue, Sep 23, 2014 at 02:01:41AM -0400, Tejun Heo wrote:
> On Tue, Sep 23, 2014 at 07:59:24AM +0200, Christoph Hellwig wrote:
> > "[PATCHSET percpu/for-3.18] percpu_ref: implement switch_to_atomic/percpu()"
> > 
> > looks way to big for 3.17, and the regression was introduced in the 3.17
> > merge window.  I'm not sure what was broken before, but it defintively
> > survived a lot of testing.
> 
> Do we even care about fixing it for 3.17?  scsi-mq isn't enabled by
> default even for 3.18.  The open-coded percpu ref thing was subtly
> broken there.  It'd be difficult to trigger but I'm fairly sure it'd
> crap out in the wild once in a blue moon.

Hmmm... also, this is actually an issue with scsi that block layer is
working around.  For other drivers (virtio), the latency during
shutdown should be completely fine.

-- 
tejun
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