linux-kernel - Re: [PATCH 07/12] cfq-iosched: implement hierarchy-ready cfq

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Date:	Mon, 17 Dec 2012 14:12:54 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	lizefan@...wei.com, axboe@...nel.dk,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, ctalbott@...gle.com, rni@...gle.com
Subject: Re: [PATCH 07/12] cfq-iosched: implement hierarchy-ready cfq_group
 charge scaling

Hey, Vivek.

On Mon, Dec 17, 2012 at 04:49:11PM -0500, Vivek Goyal wrote:
> > Eh?  Then how would it compete with the children cfqgs?  You can
> > consider it as the hidden leaf node competing with the children cfqgs,
> > right?  So, you take the leaf weight and divide it by the total active
> > weight of all the children (including the hidden leaf node).  It isn't
> > different from the rest of the calculation.  It just looks different
> > because leaf_weight is stored elsewhere and we look down there and
> > then walke up.  The calculation being done is exactly the same one.
> 
> Again it is coming from multiplexing cfqg and cfqg->task_group. So
> effectively we are calculating the vfraction of task_group inside cfqg.

Or calculating the fraction of the hidden leaf node.

> Once you say cfqg->vfraction, I immediately think of total share of
> group (including task_group and all children cgroups).

Any tasks should be treated as member of the hidden leaf node, so...

> May be cfqg->task_group_vfraction (or cfqg->tg_fraction) is a better name.

I don't know.  Wouldn't just explaining clearly in the comment e
enough?  I think the name "vfraction" is already weird enough to
indicate that it's something which you need to read explanation about.
If that explanation is readily available as comment, I think it should
be okay.

> Also descrition says.
> 
> /* vfraction  the fraction of vdisktime that a cfqg is entitled to */
> 
> May be we can clarify it that it is share of task_group of cfqg.

Yeah, no objection.  Let's beef up the explanation there.

> I guess i am too familiar with how cpu has done it. I think I am getting
> confused with properties which belong to cfqg and properties which
> belong to cfqg->task_group.
> 
> I guess if we prefix fields belong to task_group with somete suitable
> string, it might become little easier to understand.

I usually am not a big fan of adding wordy prefixes.  I think they
tend to obfuscate/frustrate more than help.  Let's stick a weird (that
is, non-generic) name to something weird and explain it well in the
comment.

Thanks.

-- 
tejun
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