Message-ID: <4D08680C.5000905@cn.fujitsu.com>
Date:	Wed, 15 Dec 2010 15:02:36 +0800
From:	Gui Jianfeng <guijianfeng@...fujitsu.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	Jens Axboe <axboe@...nel.dk>, Corrado Zoccolo <czoccolo@...il.com>,
	Chad Talbott <ctalbott@...gle.com>,
	Nauman Rafique <nauman@...gle.com>,
	Divyesh Shah <dpshah@...gle.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/8 v2] cfq-iosched: Introduce hierarchical scheduling
 with CFQ queue and group at the same level

Vivek Goyal wrote:
> On Mon, Dec 13, 2010 at 09:45:01AM +0800, Gui Jianfeng wrote:
>> This patch makes CFQ queue and CFQ group schedules at the same level.
>> Consider the following hierarchy:
>>
>>                     Root
>>                    / | \
>>                  q1 q2 G1
>>                       / \
>>                     q3  G2
>>
>> q1 q2 and q3 are CFQ queues G1 and G2 are CFQ groups. With this patch, q1,
>> q2 and G1 are scheduling on a same service tree in Root CFQ group. q3 and G2
>> are schedluing under G1. Note, for the time being, CFQ group is treated
>> as "BE and SYNC" workload, and is put on "BE and SYNC" service tree. That
>> means service differentiate only happens in "BE and SYNC" service tree.
>> Later, we may introduce "IO Class" for CFQ group.
>>
>> Signed-off-by: Gui Jianfeng <guijianfeng@...fujitsu.com>
>> ---
>>  block/cfq-iosched.c |  473 ++++++++++++++++++++++++++++++++++----------------
>>  1 files changed, 321 insertions(+), 152 deletions(-)
>>
>> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
>> index 6486956..d90627e 100644
>> --- a/block/cfq-iosched.c
>> +++ b/block/cfq-iosched.c
>> @@ -105,6 +105,9 @@ struct cfq_entity {
>>  	u64 vdisktime;
>>  	bool is_group_entity;
>>  	unsigned int weight;
>> +	struct cfq_entity *parent;
>> +	/* Reposition time */
>> +	unsigned long reposition_time;
>>  };
>>  
>>  /*
>> @@ -113,8 +116,6 @@ struct cfq_entity {
>>  struct cfq_queue {
>>  	/* The schedule entity */
>>  	struct cfq_entity cfqe;
>> -	/* Reposition time */
>> -	unsigned long reposition_time;
>>  	/* reference count */
>>  	atomic_t ref;
>>  	/* various state flags, see below */
>> @@ -194,6 +195,9 @@ struct cfq_group {
>>  	/* number of cfqq currently on this group */
>>  	int nr_cfqq;
>>  
>> +	/* number of sub cfq groups */
>> +	int nr_subgp;
>> +
> 
> Do you really have to maintain separate count for child queue and
> child groups? Will a common count something like nr_children be 
> not sufficient.

Currently, nr_subgp is only effective in hierarchical mode, but nr_cfqq work
for both hierarchical and flat mode. So for the time being, we need separate
count for child queues and groups.

> 
>>  	/*
>>  	 * Per group busy queus average. Useful for workload slice calc. We
>>  	 * create the array for each prio class but at run time it is used
>> @@ -229,8 +233,6 @@ struct cfq_group {
>>   */
>>  struct cfq_data {
>>  	struct request_queue *queue;
>> -	/* Root service tree for cfq_groups */
>> -	struct cfq_rb_root grp_service_tree;
> 
> I see that you are removing this service tree here and then adding it
> back in patch 7. I think it is confusing. In fact title of patch 7 is
> add flat mode. The fact the flat mode is already supported and we
> are just adding hierarchical mode on top of it. I think this is
> just a matter of better naming and patch organization so that 
> it is more clear.  

Ok, will merge hierarchical patch and flat patch together.

> 
>>  	struct cfq_group root_group;
>>  
>>  	/*
>> @@ -347,8 +349,6 @@ cfqg_of_entity(struct cfq_entity *cfqe)
>>  	return NULL;
>>  }
>>  
>> -static struct cfq_group *cfq_get_next_cfqg(struct cfq_data *cfqd);
>> -
>>  static struct cfq_rb_root *service_tree_for(struct cfq_group *cfqg,
>>  					    enum wl_prio_t prio,
>>  					    enum wl_type_t type)
>> @@ -638,10 +638,15 @@ static inline unsigned cfq_group_get_avg_queues(struct cfq_data *cfqd,
>>  static inline unsigned
>>  cfq_group_slice(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_rb_root *st = cfqe->service_tree;
>>  
>> -	return cfq_target_latency * cfqe->weight / st->total_weight;
>> +	if (st)
>> +		return cfq_target_latency * cfqe->weight
>> +			/ st->total_weight;
> 
> Is it still true in hierarchical mode. Previously group used to be
> at the top and there used to be only one service tree for groups so
> st->total_weight represented total weight in the system.
>  
> Now with hierarhcy this will not/should not be true. So a group slice
> calculation should be different?

I just keep the original group slice calculation here. I was thinking that
calculate group slice in a hierachical way, this might get a really small
group slice and not sure how it works. So I just keep the original calculation.
Any thoughts?

> 
>> +	else
>> +		/* If this is the root group, give it a full slice. */
>> +		return cfq_target_latency;
>>  }
>>  
>>  static inline void
>> @@ -804,17 +809,6 @@ static struct cfq_entity *cfq_rb_first(struct cfq_rb_root *root)
>>  	return NULL;
>>  }
>>  
>> -static struct cfq_entity *cfq_rb_first_entity(struct cfq_rb_root *root)
>> -{
>> -	if (!root->left)
>> -		root->left = rb_first(&root->rb);
>> -
>> -	if (root->left)
>> -		return rb_entry_entity(root->left);
>> -
>> -	return NULL;
>> -}
>> -
>>  static void rb_erase_init(struct rb_node *n, struct rb_root *root)
>>  {
>>  	rb_erase(n, root);
>> @@ -888,12 +882,15 @@ __cfq_entity_service_tree_add(struct cfq_rb_root *st, struct cfq_entity *cfqe)
>>  
>>  	rb_link_node(&cfqe->rb_node, parent, node);
>>  	rb_insert_color(&cfqe->rb_node, &st->rb);
>> +
>> +	update_min_vdisktime(st);
>>  }
>>  
>>  static void
>>  cfq_entity_service_tree_add(struct cfq_rb_root *st, struct cfq_entity *cfqe)
>>  {
>>  	__cfq_entity_service_tree_add(st, cfqe);
>> +	cfqe->reposition_time = jiffies;
>>  	st->count++;
>>  	st->total_weight += cfqe->weight;
>>  }
>> @@ -901,34 +898,57 @@ cfq_entity_service_tree_add(struct cfq_rb_root *st, struct cfq_entity *cfqe)
>>  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++;
>> +
>> +	/*
>> +	 * Root group doesn't belongs to any service
>> +	 */
>> +	if (cfqg == &cfqd->root_group)
>> +		return;
> 
> Can we keep root group on cfqd->grp_service_tree?  In hierarchical mode
> there will be only 1 group on grp service tree and in flat mode there
> can be many.

Keep top service tree different for hierarchical mode and flat mode is just
fine to me. If you don't strongly object, I'd to keep the current way. :)

> 
>> +
>>  	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 && cfqe->parent) {
>> +		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.
>> +		 */
>> +		st = cfqe->service_tree;
>> +		n = rb_last(&st->rb);
>> +		if (n) {
>> +			entity = rb_entry_entity(n);
>> +			cfqe->vdisktime = entity->vdisktime +
>> +				CFQ_IDLE_DELAY;
>> +		} else
>> +			cfqe->vdisktime = st->min_vdisktime;
>>  
>> -	cfq_entity_service_tree_add(st, cfqe);
>> +		cfq_entity_service_tree_add(st, cfqe);
>> +		cfqe = cfqe->parent;
>> +		__cfqg = cfqg_of_entity(cfqe);
>> +		__cfqg->nr_subgp++;
>> +	}
>>  }
>>  
>>  static void
>>  __cfq_entity_service_tree_del(struct cfq_rb_root *st, struct cfq_entity *cfqe)
>>  {
>>  	cfq_rb_erase(&cfqe->rb_node, st);
>> +	update_min_vdisktime(st);
>>  }
>>  
>>  static void
>> @@ -937,27 +957,47 @@ cfq_entity_service_tree_del(struct cfq_rb_root *st, struct cfq_entity *cfqe)
>>  	if (!RB_EMPTY_NODE(&cfqe->rb_node)) {
>>  		__cfq_entity_service_tree_del(st, cfqe);
>>  		st->total_weight -= cfqe->weight;
>> -		cfqe->service_tree = NULL;
>>  	}
>>  }
>>  
>>  static void
>>  cfq_group_service_tree_del(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_group *__cfqg, *p_cfqg;
>>  
>>  	BUG_ON(cfqg->nr_cfqq < 1);
>>  	cfqg->nr_cfqq--;
>>  
>> +	/*
>> +	 * Root group doesn't belongs to any service
>> +	 */
>> +	if (cfqg == &cfqd->root_group)
>> +		return;
>> +
>>  	/* If there are other cfq queues under this group, don't delete it */
>>  	if (cfqg->nr_cfqq)
>>  		return;
>>  
>> -	cfq_log_cfqg(cfqd, cfqg, "del_from_rr group");
>> -	cfq_entity_service_tree_del(st, cfqe);
>> -	cfqg->saved_workload_slice = 0;
>> -	cfq_blkiocg_update_dequeue_stats(&cfqg->blkg, 1);
>> +	/* If child group exists, don't dequeue it */
>> +	if (cfqg->nr_subgp)
>> +		return;
>> +
>> +	/*
>> +	 * Dequeue this group and its ancestors from their service tree.
>> +	 */
>> +	while (cfqe && cfqe->parent) {
>> +		__cfqg = cfqg_of_entity(cfqe);
>> +		p_cfqg = cfqg_of_entity(cfqe->parent);
>> +		cfq_entity_service_tree_del(cfqe->service_tree, cfqe);
>> +		cfq_blkiocg_update_dequeue_stats(&__cfqg->blkg, 1);
>> +		cfq_log_cfqg(cfqd, __cfqg, "del_from_rr group");
>> +		__cfqg->saved_workload_slice = 0;
>> +		cfqe = cfqe->parent;
>> +		p_cfqg->nr_subgp--;
>> +		if (p_cfqg->nr_cfqq || p_cfqg->nr_subgp)
>> +			return;
>> +	}
>>  }
> 
> I think one you merge queue/group algorithms, you can use same function
> for adding/deleting queue/group entities and don't have to use separate
> functions for groups?

The CFQ entity adding/deleting for queue/group are almost the same, and I'v
already extract common function to handle it.
cfq_entity_service_tree_add() and cfq_entity_service_tree_del()

> 
> [..]
>> -	cfqg->cfqe.weight = blkcg_get_weight(blkcg, cfqg->blkg.dev);
>> +int cfqg_chain_alloc(struct cfq_data *cfqd, struct cgroup *cgroup)
>> +{
>> +	struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup);
>> +	struct blkio_cgroup *p_blkcg;
>> +	struct backing_dev_info *bdi = &cfqd->queue->backing_dev_info;
>> +	unsigned int major, minor;
>> +	struct cfq_group *cfqg, *p_cfqg;
>> +	void *key = cfqd;
>> +	int ret;
>>  
>> -	/* Add group on cfqd list */
>> -	hlist_add_head(&cfqg->cfqd_node, &cfqd->cfqg_list);
>> +	cfqg = cfqg_of_blkg(blkiocg_lookup_group(blkcg, key));
>> +	if (cfqg) {
>> +		if (!cfqg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {
>> +			sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
>> +			cfqg->blkg.dev = MKDEV(major, minor);
>> +		}
>> +		/* chain has already been built */
>> +		return 0;
>> +	}
>> +
>> +	cfqg = kzalloc_node(sizeof(*cfqg), GFP_ATOMIC, cfqd->queue->node);
>> +	if (!cfqg)
>> +		return -1;
>> +
>> +	init_cfqg(cfqd, blkcg, cfqg);
>> +
>> +	/* Already to the top group */
>> +	if (!cgroup->parent)
>> +		return 0;
>> +
>> +	/* Allocate CFQ groups on the chain */
>> +	ret = cfqg_chain_alloc(cfqd, cgroup->parent);
> 
> Can you avoid recursion and user for/while loops to initialize the
> chain. Don't want to push multiple stack frames in case of a deep hier.
> 

OK, will change.

>> +	if (ret == -1) {
>> +		uninit_cfqg(cfqd, cfqg);
>> +		return -1;
>> +	}
>> +
>> +	p_blkcg = cgroup_to_blkio_cgroup(cgroup->parent);
>> +	p_cfqg = cfqg_of_blkg(blkiocg_lookup_group(p_blkcg, key));
>> +	BUG_ON(p_cfqg == NULL);
>> +
>> +	cfqg_set_parent(cfqd, cfqg, p_cfqg);
>> +	return 0;
>> +}
>> +
>> +static struct cfq_group *
>> +cfq_find_alloc_cfqg(struct cfq_data *cfqd, struct cgroup *cgroup, int create)
>> +{
>> +	struct blkio_cgroup *blkcg = cgroup_to_blkio_cgroup(cgroup);
>> +	struct cfq_group *cfqg = NULL;
>> +	void *key = cfqd;
>> +	struct backing_dev_info *bdi = &cfqd->queue->backing_dev_info;
>> +	unsigned int major, minor;
>> +	int ret;
>> +
>> +	cfqg = cfqg_of_blkg(blkiocg_lookup_group(blkcg, key));
>> +	if (cfqg && !cfqg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {
>> +		sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
>> +		cfqg->blkg.dev = MKDEV(major, minor);
>> +		goto done;
>> +	}
>> +	if (cfqg || !create)
>> +		goto done;
>> +
>> +	/*
>> +	 * For hierarchical cfq group scheduling, we need to allocate
>> +	 * the whole cfq group chain.
>> +	 */
>> +	ret = cfqg_chain_alloc(cfqd, cgroup);
>> +	if (!ret) {
>> +		cfqg = cfqg_of_blkg(blkiocg_lookup_group(blkcg, key));
>> +		BUG_ON(cfqg == NULL);
>> +		goto done;
>> +	}
>>  
>>  done:
>>  	return cfqg;
>> @@ -1140,12 +1278,22 @@ static void cfq_put_cfqg(struct cfq_group *cfqg)
>>  {
>>  	struct cfq_rb_root *st;
>>  	int i, j;
>> +	struct cfq_entity *cfqe;
>> +	struct cfq_group *p_cfqg;
>>  
>>  	BUG_ON(atomic_read(&cfqg->ref) <= 0);
>>  	if (!atomic_dec_and_test(&cfqg->ref))
>>  		return;
>>  	for_each_cfqg_st(cfqg, i, j, st)
>>  		BUG_ON(!RB_EMPTY_ROOT(&st->rb));
>> +
>> +	cfqe = &cfqg->cfqe;
>> +	if (cfqe->parent) {
>> +		p_cfqg = cfqg_of_entity(cfqe->parent);
>> +		/* Drop the reference taken by children */
>> +		atomic_dec(&p_cfqg->ref);
>> +	}
>> +
> 
> Is it the right way to free up whole of the parent chain. just think that
> in a hier of test1->test2->test3 somebody drops the reference to test3
> and test1 and test2 don't have any other children. In that case after
> freeing up test3, we should be freeing up test2 and test1 also. 
> 
> I was thiking that we can achieve this by freeing up groups in a
> loop.
> 
> do {
> 	cfqe = cfqg->entity.parent;
> 	if (!atomic_dec_and_test(&cfqg->ref))
> 		return;
> 	kfree(cfqg);
> 	cfqg = cfqg_of_entity(cfqe);	
> } while(cfqg)
> 

OK

>>  	kfree(cfqg);
>>  }
>>  
>> @@ -1358,8 +1506,6 @@ insert:
>>  	/* Add cfqq onto service tree. */
>>  	cfq_entity_service_tree_add(service_tree, cfqe);
>>  
>> -	update_min_vdisktime(service_tree);
>> -	cfqq->reposition_time = jiffies;
>>  	if ((add_front || !new_cfqq) && !group_changed)
>>  		return;
>>  	cfq_group_service_tree_add(cfqd, cfqq->cfqg);
>> @@ -1802,28 +1948,30 @@ static struct cfq_queue *cfq_get_next_queue(struct cfq_data *cfqd)
>>  	return cfqq_of_entity(cfq_rb_first(service_tree));
>>  }
>>  
>> -static struct cfq_queue *cfq_get_next_queue_forced(struct cfq_data *cfqd)
>> +static struct cfq_entity *
>> +cfq_get_next_entity_forced(struct cfq_data *cfqd, struct cfq_group *cfqg)
>>  {
>> -	struct cfq_group *cfqg;
>> -	struct cfq_entity *cfqe;
>> +	struct cfq_entity *entity;
>>  	int i, j;
>>  	struct cfq_rb_root *st;
>>  
>>  	if (!cfqd->rq_queued)
>>  		return NULL;
>>  
>> -	cfqg = cfq_get_next_cfqg(cfqd);
>> -	if (!cfqg)
>> -		return NULL;
>> -
>>  	for_each_cfqg_st(cfqg, i, j, st) {
>> -		cfqe = cfq_rb_first(st);
>> -		if (cfqe != NULL)
>> -			return cfqq_of_entity(cfqe);
>> +		entity = cfq_rb_first(st);
>> +
>> +		if (entity && !entity->is_group_entity)
>> +			return entity;
>> +		else if (entity && entity->is_group_entity) {
>> +			cfqg = cfqg_of_entity(entity);
>> +			return cfq_get_next_entity_forced(cfqd, cfqg);
>> +		}
>>  	}
>>  	return NULL;
>>  }
> 
> Can above be siplified by just taking cfqd as parameter. Will work both
> for hierarchical and flat mode. Wanted to avoid recursion as somebody
> can create deep cgroup hierarchy and push lots of frames on stack.

Ok, will change.

> 
> struct cfq_entity *cfq_get_next_entity_forced(struct cfq_data *cfqd)
> {
> 	struct service_tree *st = cfqd->grp_service_tree;
> 
> 	do {
> 		cfqe = cfq_rb_first(st);
> 		if (is_cfqe_cfqq(cfqe))
> 			return cfqe;
> 		st = choose_service_tree_forced(cfqg);
> 	} while (st);
> }
> 
> And choose_service_tree_forced() can be something like.
> 
> choose_service_tree_forced(cfqg) {
> 	for_each_cfqg_st() {
> 		if (st->count !=0)
> 			return st;
> 	}
> }
> 
>>  
>> +
>>  /*
>>   * Get and set a new active queue for service.
>>   */
>> @@ -2179,7 +2327,6 @@ static enum wl_type_t cfq_choose_wl(struct cfq_data *cfqd,
>>  				struct cfq_group *cfqg, enum wl_prio_t prio)
>>  {
>>  	struct cfq_entity *cfqe;
>> -	struct cfq_queue *cfqq;
>>  	unsigned long lowest_start_time;
>>  	int i;
>>  	bool time_valid = false;
>> @@ -2191,10 +2338,9 @@ static enum wl_type_t cfq_choose_wl(struct cfq_data *cfqd,
>>  	 */
>>  	for (i = 0; i <= SYNC_WORKLOAD; ++i) {
>>  		cfqe = cfq_rb_first(service_tree_for(cfqg, prio, i));
>> -		cfqq = cfqq_of_entity(cfqe);
>>  		if (cfqe && (!time_valid ||
>> -			     cfqq->reposition_time < lowest_start_time)) {
>> -			lowest_start_time = cfqq->reposition_time;
>> +			     cfqe->reposition_time < lowest_start_time)) {
>> +			lowest_start_time = cfqe->reposition_time;
>>  			cur_best = i;
>>  			time_valid = true;
>>  		}
>> @@ -2203,47 +2349,13 @@ static enum wl_type_t cfq_choose_wl(struct cfq_data *cfqd,
>>  	return cur_best;
>>  }
>>  
>> -static void choose_service_tree(struct cfq_data *cfqd, struct cfq_group *cfqg)
>> +static void set_workload_expire(struct cfq_data *cfqd, struct cfq_group *cfqg)
>>  {
>>  	unsigned slice;
>>  	unsigned count;
>>  	struct cfq_rb_root *st;
>>  	unsigned group_slice;
>>  
>> -	if (!cfqg) {
>> -		cfqd->serving_prio = IDLE_WORKLOAD;
>> -		cfqd->workload_expires = jiffies + 1;
>> -		return;
>> -	}
>> -
>> -	/* Choose next priority. RT > BE > IDLE */
>> -	if (cfq_group_busy_queues_wl(RT_WORKLOAD, cfqd, cfqg))
>> -		cfqd->serving_prio = RT_WORKLOAD;
>> -	else if (cfq_group_busy_queues_wl(BE_WORKLOAD, cfqd, cfqg))
>> -		cfqd->serving_prio = BE_WORKLOAD;
>> -	else {
>> -		cfqd->serving_prio = IDLE_WORKLOAD;
>> -		cfqd->workload_expires = jiffies + 1;
>> -		return;
>> -	}
>> -
>> -	/*
>> -	 * For RT and BE, we have to choose also the type
>> -	 * (SYNC, SYNC_NOIDLE, ASYNC), and to compute a workload
>> -	 * expiration time
>> -	 */
>> -	st = service_tree_for(cfqg, cfqd->serving_prio, cfqd->serving_type);
>> -	count = st->count;
>> -
>> -	/*
>> -	 * check workload expiration, and that we still have other queues ready
>> -	 */
>> -	if (count && !time_after(jiffies, cfqd->workload_expires))
>> -		return;
>> -
>> -	/* otherwise select new workload type */
>> -	cfqd->serving_type =
>> -		cfq_choose_wl(cfqd, cfqg, cfqd->serving_prio);
>>  	st = service_tree_for(cfqg, cfqd->serving_prio, cfqd->serving_type);
>>  	count = st->count;
>>  
>> @@ -2284,26 +2396,51 @@ static void choose_service_tree(struct cfq_data *cfqd, struct cfq_group *cfqg)
>>  	cfqd->workload_expires = jiffies + slice;
>>  }
>>  
>> -static struct cfq_group *cfq_get_next_cfqg(struct cfq_data *cfqd)
>> +static void choose_service_tree(struct cfq_data *cfqd, struct cfq_group *cfqg)
>>  {
>> -	struct cfq_rb_root *st = &cfqd->grp_service_tree;
>> -	struct cfq_group *cfqg;
>> -	struct cfq_entity *cfqe;
>> +	struct cfq_rb_root *st;
>> +	unsigned count;
>>  
>> -	if (RB_EMPTY_ROOT(&st->rb))
>> -		return NULL;
>> -	cfqe = cfq_rb_first_entity(st);
>> -	cfqg = cfqg_of_entity(cfqe);
>> -	BUG_ON(!cfqg);
>> -	update_min_vdisktime(st);
>> -	return cfqg;
>> +	if (!cfqg) {
>> +		cfqd->serving_prio = IDLE_WORKLOAD;
>> +		cfqd->workload_expires = jiffies + 1;
>> +		return;
>> +	}
> 
> I am wondering where do we use above code. Do we ever call
> choose_service_tree() with cfqg == NULL? Can't seem to figure out by
> looking at the code.

Already fixed by a seperate patch. ;)

> 
>> +
>> +	/* Choose next priority. RT > BE > IDLE */
>> +	if (cfq_group_busy_queues_wl(RT_WORKLOAD, cfqd, cfqg))
>> +		cfqd->serving_prio = RT_WORKLOAD;
>> +	else if (cfq_group_busy_queues_wl(BE_WORKLOAD, cfqd, cfqg))
>> +		cfqd->serving_prio = BE_WORKLOAD;
>> +	else {
>> +		cfqd->serving_prio = IDLE_WORKLOAD;
>> +		cfqd->workload_expires = jiffies + 1;
>> +		return;
>> +	}
>> +
>> +	/*
>> +	 * For RT and BE, we have to choose also the type
>> +	 * (SYNC, SYNC_NOIDLE, ASYNC), and to compute a workload
>> +	 * expiration time
>> +	 */
>> +	st = service_tree_for(cfqg, cfqd->serving_prio, cfqd->serving_type);
>> +	count = st->count;
>> +
>> +	/*
>> +	 * check workload expiration, and that we still have other queues ready
>> +	 */
>> +	if (count && !time_after(jiffies, cfqd->workload_expires))
>> +		return;
>> +
>> +	/* otherwise select new workload type */
>> +	cfqd->serving_type =
>> +		cfq_choose_wl(cfqd, cfqg, cfqd->serving_prio);
>>  }
>>  
>> -static void cfq_choose_cfqg(struct cfq_data *cfqd)
>> +struct cfq_entity *choose_serving_entity(struct cfq_data *cfqd,
>> +					      struct cfq_group *cfqg)
> 
> I think for this function you don't have to pass cfqg as parameter. You
> can just use cfqd as parameter and then take all decisions based on
> service tree.
> 
> So at top we can continue to have grp_service_tree in cfqd. In
> hierarchical mode it will only have root group queued there and in
> flat mode it can have multiple groups queued.
> 
> Also I am looking forward to simplifying and organizing CFQ code little
> better so that it is easy to read. Can chooser_serving entity function
> be organized something as follows. This shoudl work both for flat and
> hierarchical modes. Following is only a pesudo code.
> 
> struct cfq_entity *select_entity(struct cfq_data *cfqd)
> {
> 	struct cfq_rb_root *st = cfqd->grp_service_tree;
> 	struct cfq_entity *cfqe;
> 
> 	do {
> 		cfqe = cfq_rb_first(st);
> 		if (is_cfqe_cfqq(cfqe))
> 			/* We found the next queue to dispatch from */
> 			break;
> 		else
> 		 	st = choose_service_tree();			
> 	} while (st)
> 
> 	return cfqe;
> }
> 

Ok, I'll refine the code to make it easier to read.

>>  {
>> -	struct cfq_group *cfqg = cfq_get_next_cfqg(cfqd);
>> -
>> -	cfqd->serving_group = cfqg;
>> +	struct cfq_rb_root *service_tree;
>>  
>>  	/* Restore the workload type data */
>>  	if (cfqg->saved_workload_slice) {
>> @@ -2314,8 +2451,21 @@ static void cfq_choose_cfqg(struct cfq_data *cfqd)
>>  		cfqd->workload_expires = jiffies - 1;
>>  
>>  	choose_service_tree(cfqd, cfqg);
>> -}
>>  
>> +	service_tree = service_tree_for(cfqg, cfqd->serving_prio,
>> +					cfqd->serving_type);
>> +
>> +	if (!cfqd->rq_queued)
>> +		return NULL;
>> +
>> +	/* There is nothing to dispatch */
>> +	if (!service_tree)
>> +		return NULL;
>> +	if (RB_EMPTY_ROOT(&service_tree->rb))
>> +		return NULL;
>> +
>> +	return cfq_rb_first(service_tree);
>> +}
>>  /*
>>   * Select a queue for service. If we have a current active queue,
>>   * check whether to continue servicing it, or retrieve and set a new one.
>> @@ -2323,6 +2473,8 @@ static void cfq_choose_cfqg(struct cfq_data *cfqd)
>>  static struct cfq_queue *cfq_select_queue(struct cfq_data *cfqd)
>>  {
>>  	struct cfq_queue *cfqq, *new_cfqq = NULL;
>> +	struct cfq_group *cfqg;
>> +	struct cfq_entity *entity;
>>  
>>  	cfqq = cfqd->active_queue;
>>  	if (!cfqq)
>> @@ -2422,8 +2574,23 @@ new_queue:
>>  	 * Current queue expired. Check if we have to switch to a new
>>  	 * service tree
>>  	 */
>> -	if (!new_cfqq)
>> -		cfq_choose_cfqg(cfqd);
>> +	cfqg = &cfqd->root_group;
>> +
>> +	if (!new_cfqq) {
>> +		do {
>> +			entity = choose_serving_entity(cfqd, cfqg);
>> +			if (entity && !entity->is_group_entity) {
>> +				/* This is the CFQ queue that should run */
>> +				new_cfqq = cfqq_of_entity(entity);
>> +				cfqd->serving_group = cfqg;
>> +				set_workload_expire(cfqd, cfqg);
>> +				break;
>> +			} else if (entity && entity->is_group_entity) {
>> +				/* Continue to lookup in this CFQ group */
>> +				cfqg = cfqg_of_entity(entity);
>> +			}
>> +		} while (entity && entity->is_group_entity);
> 
> I think move above logic in a separate function otherwise select_queue()
> is becoming complicated.
> 
> Secondly for traversing the hierarchy you can introduce macros like
> for_each_entity() or for_each_cfqe() etc.
> 
> Thirdly I would again say that flat mode is already supported. Build on
> top of it instead of first getting rid of it and then adding it back
> with the help of a separate patch. If it is too complicated then let
> it be a single patch instead of separating it out in two pathes.

Ok

Thanks for reviewing Vievk, will post a updated patch.

Gui

> 
>> +	}
> 
>>  
>>  	cfqq = cfq_set_active_queue(cfqd, new_cfqq);
>>  keep_queue:
>> @@ -2454,10 +2621,14 @@ static int cfq_forced_dispatch(struct cfq_data *cfqd)
>>  {
>>  	struct cfq_queue *cfqq;
>>  	int dispatched = 0;
>> +	struct cfq_entity *entity;
>> +	struct cfq_group *root = &cfqd->root_group;
>>  
>>  	/* Expire the timeslice of the current active queue first */
>>  	cfq_slice_expired(cfqd, 0);
>> -	while ((cfqq = cfq_get_next_queue_forced(cfqd)) != NULL) {
>> +	while ((entity = cfq_get_next_entity_forced(cfqd, root)) != NULL) {
>> +		BUG_ON(entity->is_group_entity);
>> +		cfqq = cfqq_of_entity(entity);
>>  		__cfq_set_active_queue(cfqd, cfqq);
>>  		dispatched += __cfq_forced_dispatch_cfqq(cfqq);
>>  	}
>> @@ -3991,9 +4162,6 @@ static void *cfq_init_queue(struct request_queue *q)
>>  
>>  	cfqd->cic_index = i;
>>  
>> -	/* Init root service tree */
>> -	cfqd->grp_service_tree = CFQ_RB_ROOT;
>> -
>>  	/* Init root group */
>>  	cfqg = &cfqd->root_group;
>>  	for_each_cfqg_st(cfqg, i, j, st)
>> @@ -4003,6 +4171,7 @@ static void *cfq_init_queue(struct request_queue *q)
>>  	/* Give preference to root group over other groups */
>>  	cfqg->cfqe.weight = 2*BLKIO_WEIGHT_DEFAULT;
>>  	cfqg->cfqe.is_group_entity = true;
>> +	cfqg->cfqe.parent = NULL;
>>  
>>  #ifdef CONFIG_CFQ_GROUP_IOSCHED
>>  	/*
>> -- 
>> 1.6.5.2
>>
>>
> 
--
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