linux-kernel - Re: CFQ and dm-crypt

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Date:	Wed, 03 Nov 2010 16:54:42 +0100
From:	Richard Kralovic <Richard.Kralovic@....fmph.uniba.sk>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	linux-kernel@...r.kernel.org, axboe@...nel.dk
Subject: Re: CFQ and dm-crypt

On 11/03/10 04:23, Jeff Moyer wrote:
>> > CFQ io scheduler relies on using task_struct current to determine which
>> > process makes the io request. On the other hand, some dm modules (such
>> > as dm-crypt) use separate threads for doing io. As CFQ sees only these
>> > threads, it provides a very poor performance in such a case.
>> >
>> > IMHO the correct solution for this would be to store, for every io
>> > request, the process that initiated it (and preserve this information
>> > while the request is processed by device mapper). Would that be feasible?
> Sure.  Try the attached patch (still an rfc) and let us know how it
> goes.  In my environment, it speed up multiple concurrent buffered
> readers.  I wasn't able to do a full analysis via blktrace as 2.6.37-rc1
> seems to have broken blktrace support on my system.

Thanks for the patch. Unfortunately, I got a kernel panic quite soon
after booting the patched kernel. I was not able to reproduce the
panic in a virtual machine, so I had to manually note the backtrace,
thus I apologize that it's incomplete:

Fatal exception in interupt.
...
do_invalid_op
cic_free_func 0x9d/0xb0
bio_endio 0x42/0x70
task_rq_lock
try_to_wake_up
invalid_op
cic_free_func
cfq_free_io_context
put_io_context
cfq_put_request
...

When I combined the patch with my previous hack on dm-crypt, it worked
fine; so the problem apparently goes away if cfq sees the corret io
context.

Moreover, I noticed in the sources that cfq still uses current task on
many places. For example, the CPU scheduler settings are inherited if
there is no io priority set. Hence I was wondering if it does not make
more sense to store whole task_struct of the initiating process in
bio, instead of just io_context?

Greets
      Richard

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