linux-kernel - Re: [PATCH RFC - TAKE TWO - 00/12] New version of the BFQ I/O Scheduler

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Message-Id: <FCFE0106-A4DD-4DEF-AAAE-040F3823A447@unimore.it>
Date:	Tue, 3 Jun 2014 18:54:53 +0200
From:	Paolo Valente <paolo.valente@...more.it>
To:	Pavel Machek <pavel@....cz>
Cc:	Tejun Heo <tj@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Li Zefan <lizefan@...wei.com>,
	Fabio Checconi <fchecconi@...il.com>,
	Arianna Avanzini <avanzini.arianna@...il.com>,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org
Subject: Re: [PATCH RFC - TAKE TWO - 00/12] New version of the BFQ I/O Scheduler


Il giorno 02/giu/2014, alle ore 15:02, Pavel Machek <pavel@....cz> ha scritto:

> Hi!
> 
>>> Well, it's all about how to actually route the changes and in general
>>> whenever avoidable we try to avoid whole-sale code replacement
>>> especially when most of the structural code is similar like in this
>>> case.  Gradually evolving cfq to bfq is likely to take more work but
>>> I'm very positive that it'd definitely be a lot easier to merge the
>>> changes that way and people involved, including the developers and
>>> reviewers, would acquire a lot clearer picture of what's going on in
>>> the process.  For example, AFAICS, most of the heuristics added by
>> 
>> Would it make sense to merge bfq first, _then_ turn cfq into bfq, then
>> remove bfq?
>> 
>> That way
>> 
>> 1. Users like me would see improvements soon 
>> 
>> 2. BFQ would get more testing early. 
> 
> Like this: I applied patch over today's git... 
> 
> I only see last bits of panic...
> 
> Call trace:
> __bfq_bfqq_expire
> bfq_bfqq_expire
> bfq_dispatch_requests
> sci_request_fn
> ...
> EIP: T.1839+0x26
> Kernel panic - not syncing: Fatal exception in interrupt
> Shutting down cpus with NMI
> 
> ...
> 
> Will retry.
> 
> Any ideas?
> 			

We have tried to think about ways to trigger this failure, but in vain. Unfortunately, so far no user has reported any failure with this last version of bfq either. Finally, we have gone through a new static analysis, but also in this case uselessly.

So, if you are willing to retry, we have put online a version of the code filled with many BUG_ONs. I hope they can make it easier to track down the bug. The archive is here:
http://algogroup.unimore.it/people/paolo/disk_sched/debugging-patches/3.15.0-rc8-v7rc5.tgz

Should this attempt be useless as well, I will, if you do not mind, try by asking you more details about your system and reproducing your configuration as much as I can.

Thanks,
Paolo

> 						Pavel
> -- 
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html


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