linux-kernel - Re: [PATCH 5/6 v4] cfq-iosched: CFQ group hierarchical scheduling and use

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Date:	Mon, 14 Feb 2011 13:10:27 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Gui Jianfeng <guijianfeng@...fujitsu.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Shaohua Li <shaohua.li@...el.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Chad Talbott <ctalbott@...gle.com>,
	Divyesh Shah <dpshah@...gle.com>
Subject: Re: [PATCH 5/6 v4] cfq-iosched: CFQ group hierarchical scheduling
 and use_hierarchy interface

On Mon, Feb 14, 2011 at 11:20:33AM +0800, Gui Jianfeng wrote:
> Vivek Goyal wrote:
> > On Thu, Feb 10, 2011 at 03:47:45PM +0800, Gui Jianfeng wrote:
> >> CFQ group hierarchical scheduling and use_hierarchy interface.
> >>
> > 
> > Hi Gui,
> > 
> > I have done a quick high level review. Some minor comments inline.
> > 
> > [..]
> >>  struct cfq_data {
> >>  	struct request_queue *queue;
> >> -	/* Root service tree for cfq_groups */
> >> -	struct cfq_rb_root grp_service_tree;
> >>  	struct cfq_group root_group;
> >>  
> >> +	/* cfq group schedule in flat or hierarchy manner. */
> >> +	bool use_hierarchy;
> >> +
> > 
> > This seems to be redundant now? Nobody is using it?
> > 
> >>  	/*
> >>  	 * The priority currently being served
> >>  	 */
> >> @@ -246,6 +251,9 @@ struct cfq_data {
> >>  	unsigned long workload_expires;
> >>  	struct cfq_group *serving_group;
> >>  
> >> +	/* Service tree for cfq group flat scheduling mode. */
> >> +	struct cfq_rb_root grp_service_tree;
> > 
> > Above comment is misleading. This service tree is now used both for
> > flat as well as hierarhical mode.
> > 
> > [..]
> >>  static void
> >>  cfq_group_service_tree_add(struct cfq_data *cfqd, struct cfq_group *cfqg)
> >>  {
> >> -	struct cfq_rb_root *st = &cfqd->grp_service_tree;
> >>  	struct cfq_entity *cfqe = &cfqg->cfqe;
> >> -	struct cfq_entity *__cfqe;
> >>  	struct rb_node *n;
> >> +	struct cfq_entity *entity;
> >> +	struct cfq_rb_root *st;
> >> +	struct cfq_group *__cfqg;
> >>  
> >>  	cfqg->nr_cfqq++;
> >> +
> >>  	if (!RB_EMPTY_NODE(&cfqe->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.
> >> +	 * Enqueue this group and its ancestors onto their service tree.
> >>  	 */
> >> -	n = rb_last(&st->rb);
> >> -	if (n) {
> >> -		__cfqe = rb_entry_entity(n);
> >> -		cfqe->vdisktime = __cfqe->vdisktime + CFQ_IDLE_DELAY;
> >> -	} else
> >> -		cfqe->vdisktime = st->min_vdisktime;
> >> +	while (cfqe) {
> >> +		if (!RB_EMPTY_NODE(&cfqe->rb_node))
> >> +			return;
> >>  
> >> -	cfq_entity_service_tree_add(st, cfqe);
> >> +		/*
> >> +		 * 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.
> >> +		 */
> > 
> > Can we use vdisktime boost logic for groups also? I think it can be a separate
> > patch in the series (the last one). Keeping it as a separate patch will
> > also help you to coordinate with chad's patch.
> > 
> >> +		st = cfqe->service_tree;
> > 
> > Group entity set their service tree when they get allocated and retain
> > this pointer even when they get deleted from serivce tree. Queue entities
> > seem to have it NULL when they get deleted from service tree and it
> > gets set again when queue is getting inserted. It would be nice if we
> > can fix this discrepancy and keep it consistent. I think clearing up
> > cfqe->service_tree is a better idea and then calculate it again for
> > group also.
> 
> Vivek,
> 
> Currently, cfq queue might change workload type and io class, so we need to recalculate
> its service_tree. But for cfq groups, IMHO we don't need to add this complexity for the
> time being.
> I think we can add this change as soon as different io classes or workload types are
> introduced. How do you think?

Ok, that's fine.

Thanks
Vivek
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