linux-kernel - Re: [PATCH 7/9] block: implement bio_associate

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Message-ID: <20120217225125.GK26620@redhat.com>
Date:	Fri, 17 Feb 2012 17:51:26 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Kent Overstreet <koverstreet@...gle.com>, axboe@...nel.dk,
	ctalbott@...gle.com, rni@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/9] block: implement bio_associate_current()

On Fri, Feb 17, 2012 at 02:41:03PM -0800, Tejun Heo wrote:
> On Fri, Feb 17, 2012 at 05:34:20PM -0500, Vivek Goyal wrote:
> > Nope. We make note of task migration and drop cic->cfqq association
> > and establish a new association where new cfqq is part of new cgroup.
> > (ioc_cgroup_changed()).
> 
> Yeah, that's the CHANGED bit thing.  I probably got confused with
> blk-throttle losing blkcg while testing.  Anyways, it would be great
> if we can either remove that altogether.  e.g. compare the cached
> blkg->blkcg and see if it has changed from cfq.  Or add a callback and
> do the shootdown synchronously at least.

I think dropping it lazily has advantage as once the cic->cfqq association
is set, we don't worry about cgroups at all.

Otherwise on every IO, we will end up comparing submitting tasks's
cgroup and cic/cfqq's cgroup.

Also this will create problems, if two threads sharing io context are
in two different cgroups. We will frequently end up changing the
association.

So comparing probably is not a very good idea. Doing something
synchronously might be better if you don't like CGROUP_CHANGED
bit in ioc.

Thanks
Vivek
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