linux-kernel - Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

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Date:	Thu, 13 Dec 2007 20:53:50 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Mark Lord <liml@....ca>
Cc:	Mark Lord <lkml@....ca>, Matthew Wilcox <matthew@....cx>,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

On Thu, Dec 13 2007, Mark Lord wrote:
> Jens Axboe wrote:
> >On Thu, Dec 13 2007, Mark Lord wrote:
> >>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 sounds more harsh than intended --> the earlier 2.6.24 kernels (up to
> >>the first couple of -rc* ones failed here because of incompatibilities
> >>between the block/bio changes and libata.
> >>
> >>That's better, I think! 
> >
> >No worries, I didn't pick it up as harsh just as an odd conclusion :-)
> >
> >If I were you, I'd just start from the first -rc that booted for you. If
> >THAT has the bug, then we'll think of something else. If you don't get
> >anywhere, I can run some tests tomorrow and see if I can reproduce it
> >here.
> ..
> 
> I believe that *anyone* can reproduce it, since it's broken long before
> the requests ever get to SCSI or libata.  Which also means that *anyone*
> who wants to can bisect it, as well.
> 
> I don't do "bisects".

It was just a suggestion on how to narrow it down, do as you see fit.

> But I will dig a bit more and see if I can find the culprit.

Sure, I'll dig around as well.

-- 
Jens Axboe

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