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

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Message-ID: <47618B0B.8020203@rtr.ca>
Date:	Thu, 13 Dec 2007 14:42:03 -0500
From:	Mark Lord <liml@....ca>
To:	Jens Axboe <jens.axboe@...cle.com>
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 ?

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".

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

Cheers

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