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From: "Elliott, Robert (Server Storage)" <Elliott@hp.com>
To: Tejun Heo <tj@kernel.org>, Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>
Subject: RE: boot stall regression due to blk-mq: use percpu_ref for mq usage count
Date: Tue, 23 Sep 2014 14:57:48 +0000	[thread overview]
Message-ID: <94D0CD8314A33A4D9D801C0FE68B402958C94862@G9W0745.americas.hpqcorp.net> (raw)
In-Reply-To: <20140923061152.GI11740@mtj.dyndns.org>



> -----Original Message-----
> From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel-
> owner@vger.kernel.org] On Behalf Of Tejun Heo
> Sent: Tuesday, 23 September, 2014 1:12 AM
> To: Christoph Hellwig
> Cc: Jens Axboe; linux-kernel@vger.kernel.org; linux-scsi@vger.kernel.org
> Subject: Re: boot stall regression due to blk-mq: use percpu_ref for mq usage
> count
> 
> On Tue, Sep 23, 2014 at 08:09:06AM +0200, Christoph Hellwig wrote:
> > 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.
> >
> > It's compiled in by default, and people are extremly eager to test it.
> 
> Ugh, I don't know.  It's not like we have a very good baseline we can
> go back to and reverting it for -stable and then redoing it seems
> kinda excessive for a yet experimental feature.  Jens?

scsi_mod.use_blk_mq is not listed in 3.17rc6 Documentation/kernel-
parameters.txt or Documentation/scsi/scsi-parameters.txt (which
does admit "This document may not be entirely up to date and
comprehensive.").

Perhaps a description with an "experimental" warning could be 
slipped into scsi-parameters.txt in 3.17, with plans to remove
that warning in 3.18.

---
Rob Elliott    HP Server Storage




  reply	other threads:[~2014-09-23 14:58 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-09-19 11:38 boot stall regression due to blk-mq: use percpu_ref for mq usage count Christoph Hellwig
2014-09-19 19:13 ` Jens Axboe
2014-09-23  5:55   ` Christoph Hellwig
2014-09-23  5:56     ` Tejun Heo
2014-09-23  5:57       ` Tejun Heo
2014-09-23  5:59       ` Christoph Hellwig
2014-09-23  6:01         ` Tejun Heo
2014-09-23  6:03           ` Tejun Heo
2014-09-23  6:09           ` Christoph Hellwig
2014-09-23  6:11             ` Tejun Heo
2014-09-23 14:57               ` Elliott, Robert (Server Storage) [this message]
2014-09-23 15:49               ` Jens Axboe
2014-09-23 19:24                 ` [PATCH block/for-3.17-fixes/core] blk-mq, percpu_ref: implement a kludge for SCSI blk-mq stall during probe Tejun Heo
2014-09-23 19:29                   ` Jens Axboe
2014-09-23 19:48                     ` Tejun Heo
2014-09-24  8:23                   ` Christoph Hellwig
2014-09-24 14:30                     ` Jens Axboe
2014-09-24 14:33                       ` Tejun Heo
2014-09-24 14:33                         ` Jens Axboe
2014-09-24 17:20                           ` [PATCH percpu/for-3.18] Revert "blk-mq, percpu_ref: implement a kludge for SCSI blk-mq stall during probe" Tejun Heo
2014-09-23  6:08 ` [PATCH block/for-3.18/core] blk-mq: start q->mq_usage_counter in atomic mode Tejun Heo
2014-09-24 17:33   ` Tejun Heo
2014-09-24 17:43     ` [PATCH percpu/for-3.18] blk-mq, percpu_ref: " Tejun Heo

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