linux-kernel - Re: [PATCH 2/2] blk-throttle: Take blkcg->lock while traversing blkcg->policy

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Date:	Tue, 25 Oct 2011 15:01:51 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] blk-throttle: Take blkcg->lock while traversing
 blkcg->policy_list

On Tue, Oct 25, 2011 at 04:13:11PM +0200, Jens Axboe wrote:
> On 2011-10-21 14:10, Vivek Goyal wrote:
> > On Thu, Oct 20, 2011 at 02:29:58PM -0700, Tejun Heo wrote:
> >> Hello,
> >>
> >> On Thu, Oct 20, 2011 at 05:20:21PM -0400, Vivek Goyal wrote:
> >>> The only problem with this approach is that it will cleanup per device
> >>> weight rules also at elevator_exit() time which is not same as device
> >>> removal and one might device to bring CFQ back on device and we will
> >>> need the rules again.
> >>
> >> I actually think removoing those rules on elevator detach would be the
> >> right thing to do.  We don't try to keep cfq setting across elevator
> >> switch.  When we're switching from cfq, we're detaching iocg policy
> >> too.  The settings going away is perfectly fine.  I actually think
> >> it's a pretty bad idea to implement ad-hoc setting persistence in
> >> kernel.  Just making sure that userland is notified is far better
> >> approach.  Userland has all the facilities to deal with this type of
> >> situations.
> >>
> >> When switching from cfq to deadline, we lose the whole proportional io
> >> control.  It's way more confusing to have lingering settings which
> >> don't do anything.
> > 
> > I am not so sure about this. Suppose tomorrow another IO sheduler starts
> > taking into account the cgroup gloabl weight or cgroup per device weight
> > to do some kind of IO prioritization, then removing the rules upon
> > changing the IO schduler will not make sense.
> > 
> > IOW, rules are per cgroup per device and not per cgroup per IO scheduler
> > and more than one IO scheduler should be able to share the rules.
> 
> FWIW, I agree with Tejun here. A switch operation is a reset, start from
> scratch. We don't preserve other per IO-scheduler settings on a switch,
> preserving _some_ settings is just confusing.

Ok. But this is more of a per queue setting (per cgroup, per device) and
not per IO scheduler one. That's a different thing that currently only CFQ
makes use of it. 

If we start looking at them just as CFQ specific weigths, then it is a 
different story. My thought process about these files was per cgroup per
device weights. 

Thanks
Vivek
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