Re: 2.6.35-rc2-git2: Reported regressions from 2.6.34

[Jens Axboe <jaxboe@fusionio.com>]

On 11/06/10 21.07, Vivek Goyal wrote:
> On Fri, Jun 11, 2010 at 11:18:47AM +0200, Jens Axboe wrote:
>> On 2010-06-11 10:55, Ingo Molnar wrote:
>>>>> Caused by the same blkiocg_update_io_add_stats() function. Bootlog and config 
>>>>> attached. Reproducible on that sha1 and with that config.
>>>>
>>>> I think I see it, the internal CFQ blkg groups are not properly
>>>> initialized... Will send a patch shortly.
>>>
>>> Cool - can test it with a short turnaround, the bug is easy to reproduce.
>>
>> Here's a nasty patch that should fix it. Not optimal, since we really
>> just want empty functions for these when cfq group scheduling is not
>> defined.
>>
>> CC'ing the guilty parties to come up with a better patch that does NOT
>> involve ifdefs in cfq-iosched.c. We want blk-cgroup.[ch] fixed up.
>> And trimming the CC list a bit.
> 
> Jens, Ingo, I am sorry for this mess.
> 
> Jens,
> 
> How about introducing "block/cfq.h" and declaring additional set of wrapper
> functions to update blkiocg stats and make these do nothing if
> CFQ_GROUP_IOSCHED=n.
> 
> For example, in linux-2.6/block/cfq.h, we can define functions as follows.
> 
> #ifdef CONFIG_CFQ_GROUP_IOSCHED
> cfq_blkiocg_update_dequeue_stats () {
> 	blkiocg_update_dequeue_stats()
> }
> #else
> cfq_blkiocg_update_dequeue_stats () {}
> #endif
> 
> Fixing it blk-cgroup.[ch] might not be best as BLK_CGROUP is set.
> Secondly, if there are other IO control policies later, they might
> want to make use of BLK_CGROUP while cfq has disabled the group io 
> scheduling.

I already tried such a patch, but it's not exactly pretty. How about
splitting blk-cgroup.c into two parts, one that is built for
BLK_CGROUP and an additional one that is also built for
CFQ_GROUP_SCHED? Lets try and improve on the ifdef mess, not extend
it.

-- 
Jens Axboe