linux-kernel - Re: [PATCH 08/10] blkcg: implement blk-iocost

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Date:   Tue, 10 Sep 2019 09:08:55 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     hannes@...xchg.org, clm@...com, dennisz@...com, newella@...com,
        lizefan@...wei.com, axboe@...nel.dk, josef@...icpanda.com,
        Josef Bacik <jbacik@...com>, kernel-team@...com,
        Rik van Riel <riel@...riel.com>, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Valente <paolo.valente@...aro.org>
Subject: Re: [PATCH 08/10] blkcg: implement blk-iocost

Hello, Michal.

On Tue, Sep 10, 2019 at 02:55:14PM +0200, Michal Koutný wrote:
> This adds the generic io.weight attribute. How will this compose with
> the weight from IO schedulers? (AFAIK, only BFQ allows proportional
> control as of now. +CC Paolo.)

The two being enabled at the same time doesn't make sense, so we can
just switch over to bfq when bfq is selected as the iosched.  I asked
what Paolo wanted to do in terms of interface a couple times now but
didn't get an answer and he posted a patch which makes the two
controllers conflict, so....  Paolo, so it looks like you want to
rename all bfq files to drop the bfq prefix, right?  I can implement
the switching if so.

> I see this attributes are effectively per-cgroup per-device. Apparently,
> one device should have only one weight across hierarchy. Would it make
> sense to have io.bfq.weight and io.cost.weight with disjunctive devices?

It never makes sense to have both enabled, so I don't think that
interface makes sense.

> > +		.name = "cost.qos",
> > +		.flags = CFTYPE_ONLY_ON_ROOT,
> > [...]
> > +		.name = "cost.model",
> > +		.flags = CFTYPE_ONLY_ON_ROOT,
> I'm concerned that these aren't true cgroup attributes. The root cgroup
> would act as container for global configuration options. Wouldn't these
> values better fit as (configurable) attributes of the respective
> devices?

Initially, I put them under block device sysfs but it was too clumsy
with different config file formats and all.  I think it's better to
have global controller configs at the root cgroup.

> Secondly, how is CFTYPE_ONLY_ON_ROOT supposed to be presented in cgroup
> namespaces?

Not at all.  These are system-wide configs.  cgroup namespaces
shouldn't have anything which aren't in non-root cgroups.

Thanks.

-- 
tejun