linux-kernel - Re: [PATCH 2] Avoid preferential treatment of groups that aren't backlogged

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Date:	Mon, 14 Feb 2011 11:41:39 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Chad Talbott <ctalbott@...gle.com>
Cc:	jaxboe@...ionio.com, guijianfeng@...fujitsu.com, mrubin@...gle.com,
	teravest@...gle.com, jmoyer@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2] Avoid preferential treatment of groups that aren't
 backlogged

On Fri, Feb 11, 2011 at 11:39:30AM -0800, Chad Talbott wrote:

[..]
>  static struct cfq_group *cfq_get_next_cfqg(struct cfq_data *cfqd);
> @@ -873,18 +877,13 @@ cfq_group_service_tree_add(struct cfq_data *cfqd, struct cfq_group *cfqg)
>  	if (!RB_EMPTY_NODE(&cfqg->rb_node))
>  		return;
>  
> -	/*
> -	 * Currently put the group at the end. Later implement something
> -	 * so that groups get lesser vtime based on their weights, so that
> -	 * if group does not loose all if it was not continously backlogged.
> -	 */
> -	n = rb_last(&st->rb);
> -	if (n) {
> -		__cfqg = rb_entry_cfqg(n);
> -		cfqg->vdisktime = __cfqg->vdisktime + CFQ_IDLE_DELAY;
> -	} else
> +	if (cfqd->active_generation > cfqg->generation_num)
>  		cfqg->vdisktime = st->min_vdisktime;
> -
> +	else
> +		/* We assume that vdisktime was not modified when the task
> +		   was off the service tree.
> +		 */
> +		cfqg->vdisktime = max(st->min_vdisktime, cfqg->vdisktime);

I think above usage of max() also will give wrong results upon wrapping.
So how about using max_vdisktime() instead? I know that results are not
catastrophic when it happens, still lets use the right thing.

Thanks
Vivek
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