linux-kernel - Re: [PATCH 34/36] block: implement bio_associate

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Message-ID: <x494nuizmqw.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 22 Feb 2012 14:33:43 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, vgoyal@...hat.com, ctalbott@...gle.com,
	rni@...gle.com, linux-kernel@...r.kernel.org,
	Kent Overstreet <koverstreet@...gle.com>
Subject: Re: [PATCH 34/36] block: implement bio_associate_current()

Tejun Heo <tj@...nel.org> writes:

> Hey, Jeff.

Hi, Tejun!

> On Wed, Feb 22, 2012 at 08:45:02AM -0500, Jeff Moyer wrote:
>> Tejun Heo <tj@...nel.org> writes:
>> 
>> > -v2: #ifdef CONFIG_BLK_CGROUP added around bio->bi_ioc dereference in
>> >      rq_ioc() to fix build breakage.
>> 
>> This is useful for cfq without blk cgroups as well, right?  Why have you
>> limited the scope like this?
>
> Because blk-throttle is the only current user.  We can move the
> BLK_CGROUP to cover just bi_css later on as we add more users.

I guess you're going to make me read the whole patch set.  ;-)  What I'm
getting at is CFQ uses the io_context to make its scheduling decisions.
If we can propagate the issuer's I/O context from bio creation all the
way down to the I/O scheduler, then we can do a better job of accounting
I/O (and hence scheduling, preemption, etc).  As Vivek mentioned
previously, we have seen performance issues with the dm-crypt target and
CFQ, precisely because all of the I/O is submitted in the context of a
worker thread, and the the process that initiated the I/O is unknown at
that point.

Hopefully I've either cleared up my question, or proven to you that I do
need to go read the rest of the patch set to understand why my question
doesn't make sense.  Let me know which is the case.  ;-)

Cheers,
Jeff
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