Date: Thu, 13 Dec 2007 20:37:37 +0100
From: Jens Axboe <jens.axboe@oracle.com>
To: Mark Lord <liml@rtr.ca>
Cc: Mark Lord <lkml@rtr.ca>, Matthew Wilcox <matthew@wil.cx>,
       IDE/ATA development list <linux-ide@vger.kernel.org>,
       Linux Kernel <linux-kernel@vger.kernel.org>,
       linux-scsi <linux-scsi@vger.kernel.org>
Subject: Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?
Message-ID: <20071213193736.GF10104@kernel.dk>
References: <47617B92.6020908@rtr.ca> <47617C07.3020501@rtr.ca> <47617E72.6080504@rtr.ca> <20071213185326.GQ26334@parisc-linux.org> <4761821F.3050602@rtr.ca> <20071213192633.GD10104@kernel.dk> <4761883A.7050908@rtr.ca>
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On Thu, Dec 13 2007, Mark Lord wrote:
> Jens Axboe wrote:
> >On Thu, Dec 13 2007, Mark Lord wrote:
> >>Matthew Wilcox wrote:
> >>>On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> >>>>Problem confirmed.  2.6.23.8 regularly generates segments up to 64KB 
> >>>>for libata,
> >>>>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> >>>Just a suspicion ... could this be slab vs slub?  ie check your configs
> >>>are the same / similar between the two kernels.
> >>..
> >>
> >>Mmmm.. a good thought, that one.
> >>But I just rechecked, and both have CONFIG_SLAB=y
> >>
> >>My guess is that something got changed around when Jens
> >>reworked the block layer for 2.6.24.
> >>I'm going to dig around in there now.
> >
> >I didn't rework the block layer for 2.6.24 :-). The core block layer
> >changes since 2.6.23 are:
> >
> >- Support for empty barriers. Not a likely candidate.
> >- Shared tag queue fixes. Totally unlikely.
> >- sg chaining support. Not likely.
> >- The bio changes from Neil. Of the bunch, the most likely suspects in
> >  this area, since it changes some of the code involved with merges and
> >  blk_rq_map_sg().
> >- Lots of simple stuff, again very unlikely.
> >
> >Anyway, it sounds odd for this to be a block layer problem if you do see
> >occasional segments being merged. So it sounds more like the input data
> >having changed.
> >
> >Why not just bisect it?
> ..
> 
> Because the early 2.6.24 series failed to boot on this machine
> due to bugs in the block layer -- so the code that caused this regression
> is probably in the stuff from before the kernels became usable here.

That would be the sg chain stuff, I don't think there's any correlation
between the two "bugs" (ie I don't get how you jump to the conclusion
that this regression is from stuff before that). Just go back as early
as you can, you could even just start with a -rc bisect.

-- 
Jens Axboe

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