linux-kernel - Re: [PATCH 11/24] cfq-iosched: add leaf

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Date:	Tue, 8 Jan 2013 10:34:48 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
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,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH 11/24] cfq-iosched: add leaf_weight

On Fri, Dec 28, 2012 at 12:35:33PM -0800, Tejun Heo wrote:

[..]
> +
> +	/* on root, leaf_weight is mapped to weight */
> +	{
> +		.name = "leaf_weight_device",
> +		.flags = CFTYPE_ONLY_ON_ROOT,
> +		.read_seq_string = cfqg_print_weight_device,
> +		.write_string = cfqg_set_weight_device,
> +		.max_write_len = 256,
> +	},
> +	{
> +		.name = "leaf_weight",
> +		.flags = CFTYPE_ONLY_ON_ROOT,
> +		.read_seq_string = cfq_print_weight,
> +		.write_u64 = cfq_set_weight,
> +	},

Hi Tejun,

How does it help to map leaf weight to weight in root group. Old programs
anyway don't know about leaf_weight. So nobody is going to update it. And
if they update it, they better know what does it do.

I think we just need to map "weight" to "leaf_weight" once you switch to
making use of leaf_weight and at that point of time a user will expect
that updating weight retains the old behavior. I think you have done
that in later patch.

But mapping leaf_weight to weight seems unnecessary atleast from backward
compatibility point of view.

Thanks
Vivek
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