linux-kernel - Re: [PATCH 02/24] io-controller: Core of the elevator fair queuing

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Date:	Wed, 19 Aug 2009 18:01:34 +0200
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, dm-devel@...hat.com,
	jens.axboe@...cle.com, ryov@...inux.co.jp,
	balbir@...ux.vnet.ibm.com, righi.andrea@...il.com,
	nauman@...gle.com, dpshah@...gle.com, lizf@...fujitsu.com,
	mikew@...gle.com, fchecconi@...il.com, paolo.valente@...more.it,
	fernando@....ntt.co.jp, s-uchida@...jp.nec.com, taka@...inux.co.jp,
	guijianfeng@...fujitsu.com, jmoyer@...hat.com,
	dhaval@...ux.vnet.ibm.com, m-ikeda@...jp.nec.com, agk@...hat.com,
	akpm@...ux-foundation.org, peterz@...radead.org
Subject: Re: [PATCH 02/24] io-controller: Core of the elevator fair queuing

Hi Vivek,

Vivek Goyal wrote:
> o This is core of the io scheduler implemented at elevator layer. This is a mix
>   of cpu CFS scheduler and CFQ IO scheduler. Some of the bits from CFS have
>   to be derived so that we can support hierarchical scheduling. Without
>   cgroups or with-in group, we should essentially get same behavior as CFQ.
> 
> o This patch only shows non-hierarchical bits. Hierarhical code comes in later
>   patches.
> 
> o This code is the building base of introducing fair queuing logic in common
>   elevator layer so that it can be used by all the four IO schedulers.

> +static void enqueue_io_entity(struct io_entity *entity)
> +{
> +	struct io_service_tree *st = entity->st;
> +	struct io_sched_data *sd = io_entity_sched_data(entity);
> +
> +	/* In case task ioprio class changed while entity was off tree */
> +	io_entity_update_prio(entity);
> +	st->nr_active++;
> +	sd->nr_active++;
> +	entity->on_st = 1;
> +	place_entity(st, entity, 0);
> +	__enqueue_io_entity(st, entity);
> +}

> +static void put_prev_io_entity(struct io_entity *entity)
> +{
> +	struct io_service_tree *st = entity->st;
> +	struct io_sched_data *sd = io_entity_sched_data(entity);
> +
> +	st->active_entity = NULL;
> +	sd->active_entity = NULL;
> +
> +	if (unlikely(entity->ioprio_changed)) {
> +		dequeue_io_entity(entity);
> +		io_entity_update_prio(entity);

That call to io_entity_update_prio() looks redundant with the one in
enqueue_io_entity().

> +		enqueue_io_entity(entity);
> +	} else
> +		__enqueue_io_entity(st, entity);
> +}

Jerome
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