Re: [PATCH] as-iosched: update alias handling

[Jens Axboe]

On Thu, Nov 10 2005, Tejun Heo wrote:
> Unlike other ioscheds, as-iosched handles alias by chaing them using
> rq->queuelist.  As aliased requests are very rare in the first place,
> this complicates merge/dispatch handling without meaningful
> performance improvement.  This patch updates as-iosched to dump
> aliased requests into dispatch queue as other ioscheds do.
> 
> Signed-off-by: Tejun Heo <htejun@xxxxxxxxx>

In theory the way 'as' handles the aliases is faster since we postpone
pushing them to the dispatch list at the same point (and they have
strong (if not identical) locality). But it is much simpler to just
shove the offending requests onto the dispatch list.

It's really up to Nick - what do you think? Leaving patch below.

> ---
> 
> Jens, I've tested this change for several hours, but it might be
> better to postpone this change to next release.  It's your call.
> 
> diff --git a/block/as-iosched.c b/block/as-iosched.c
> --- a/block/as-iosched.c
> +++ b/block/as-iosched.c
> @@ -184,6 +184,9 @@ struct as_rq {
>  
>  static kmem_cache_t *arq_pool;
>  
> +static void as_move_to_dispatch(struct as_data *ad, struct as_rq *arq);
> +static void as_antic_stop(struct as_data *ad);
> +
>  /*
>   * IO Context helper functions
>   */
> @@ -372,7 +375,7 @@ static struct as_rq *as_find_first_arq(s
>   * existing request against the same sector), which can happen when using
>   * direct IO, then return the alias.
>   */
> -static struct as_rq *as_add_arq_rb(struct as_data *ad, struct as_rq *arq)
> +static struct as_rq *__as_add_arq_rb(struct as_data *ad, struct as_rq *arq)
>  {
>  	struct rb_node **p = &ARQ_RB_ROOT(ad, arq)->rb_node;
>  	struct rb_node *parent = NULL;
> @@ -399,6 +402,16 @@ static struct as_rq *as_add_arq_rb(struc
>  	return NULL;
>  }
>  
> +static void as_add_arq_rb(struct as_data *ad, struct as_rq *arq)
> +{
> +	struct as_rq *alias;
> +
> +	while ((unlikely(alias = __as_add_arq_rb(ad, arq)))) {
> +		as_move_to_dispatch(ad, alias);
> +		as_antic_stop(ad);
> +	}
> +}
> +
>  static inline void as_del_arq_rb(struct as_data *ad, struct as_rq *arq)
>  {
>  	if (!ON_RB(&arq->rb_node)) {
> @@ -1135,23 +1148,6 @@ static void as_move_to_dispatch(struct a
>  	/*
>  	 * take it off the sort and fifo list, add to dispatch queue
>  	 */
> -	while (!list_empty(&rq->queuelist)) {
> -		struct request *__rq = list_entry_rq(rq->queuelist.next);
> -		struct as_rq *__arq = RQ_DATA(__rq);
> -
> -		list_del(&__rq->queuelist);
> -
> -		elv_dispatch_add_tail(ad->q, __rq);
> -
> -		if (__arq->io_context && __arq->io_context->aic)
> -			atomic_inc(&__arq->io_context->aic->nr_dispatched);
> -
> -		WARN_ON(__arq->state != AS_RQ_QUEUED);
> -		__arq->state = AS_RQ_DISPATCHED;
> -
> -		ad->nr_dispatched++;
> -	}
> -
>  	as_remove_queued_request(ad->q, rq);
>  	WARN_ON(arq->state != AS_RQ_QUEUED);
>  
> @@ -1328,49 +1324,12 @@ fifo_expired:
>  }
>  
>  /*
> - * Add arq to a list behind alias
> - */
> -static inline void
> -as_add_aliased_request(struct as_data *ad, struct as_rq *arq,
> -				struct as_rq *alias)
> -{
> -	struct request  *req = arq->request;
> -	struct list_head *insert = alias->request->queuelist.prev;
> -
> -	/*
> -	 * Transfer list of aliases
> -	 */
> -	while (!list_empty(&req->queuelist)) {
> -		struct request *__rq = list_entry_rq(req->queuelist.next);
> -		struct as_rq *__arq = RQ_DATA(__rq);
> -
> -		list_move_tail(&__rq->queuelist, &alias->request->queuelist);
> -
> -		WARN_ON(__arq->state != AS_RQ_QUEUED);
> -	}
> -
> -	/*
> -	 * Another request with the same start sector on the rbtree.
> -	 * Link this request to that sector. They are untangled in
> -	 * as_move_to_dispatch
> -	 */
> -	list_add(&arq->request->queuelist, insert);
> -
> -	/*
> -	 * Don't want to have to handle merges.
> -	 */
> -	as_del_arq_hash(arq);
> -	arq->request->flags |= REQ_NOMERGE;
> -}
> -
> -/*
>   * add arq to rbtree and fifo
>   */
>  static void as_add_request(request_queue_t *q, struct request *rq)
>  {
>  	struct as_data *ad = q->elevator->elevator_data;
>  	struct as_rq *arq = RQ_DATA(rq);
> -	struct as_rq *alias;
>  	int data_dir;
>  
>  	if (arq->state != AS_RQ_PRESCHED) {
> @@ -1393,33 +1352,17 @@ static void as_add_request(request_queue
>  		atomic_inc(&arq->io_context->aic->nr_queued);
>  	}
>  
> -	alias = as_add_arq_rb(ad, arq);
> -	if (!alias) {
> -		/*
> -		 * set expire time (only used for reads) and add to fifo list
> -		 */
> -		arq->expires = jiffies + ad->fifo_expire[data_dir];
> -		list_add_tail(&arq->fifo, &ad->fifo_list[data_dir]);
> +	as_add_arq_rb(ad, arq);
> +	if (rq_mergeable(arq->request))
> +		as_add_arq_hash(ad, arq);
>  
> -		if (rq_mergeable(arq->request))
> -			as_add_arq_hash(ad, arq);
> -		as_update_arq(ad, arq); /* keep state machine up to date */
> -
> -	} else {
> -		as_add_aliased_request(ad, arq, alias);
> -
> -		/*
> -		 * have we been anticipating this request?
> -		 * or does it come from the same process as the one we are
> -		 * anticipating for?
> -		 */
> -		if (ad->antic_status == ANTIC_WAIT_REQ
> -				|| ad->antic_status == ANTIC_WAIT_NEXT) {
> -			if (as_can_break_anticipation(ad, arq))
> -				as_antic_stop(ad);
> -		}
> -	}
> +	/*
> +	 * set expire time (only used for reads) and add to fifo list
> +	 */
> +	arq->expires = jiffies + ad->fifo_expire[data_dir];
> +	list_add_tail(&arq->fifo, &ad->fifo_list[data_dir]);
>  
> +	as_update_arq(ad, arq); /* keep state machine up to date */
>  	arq->state = AS_RQ_QUEUED;
>  }
>  
> @@ -1542,23 +1485,8 @@ static void as_merged_request(request_qu
>  	 * if the merge was a front merge, we need to reposition request
>  	 */
>  	if (rq_rb_key(req) != arq->rb_key) {
> -		struct as_rq *alias, *next_arq = NULL;
> -
> -		if (ad->next_arq[arq->is_sync] == arq)
> -			next_arq = as_find_next_arq(ad, arq);
> -
> -		/*
> -		 * Note! We should really be moving any old aliased requests
> -		 * off this request and try to insert them into the rbtree. We
> -		 * currently don't bother. Ditto the next function.
> -		 */
>  		as_del_arq_rb(ad, arq);
> -		if ((alias = as_add_arq_rb(ad, arq))) {
> -			list_del_init(&arq->fifo);
> -			as_add_aliased_request(ad, arq, alias);
> -			if (next_arq)
> -				ad->next_arq[arq->is_sync] = next_arq;
> -		}
> +		as_add_arq_rb(ad, arq);
>  		/*
>  		 * Note! At this stage of this and the next function, our next
>  		 * request may not be optimal - eg the request may have "grown"
> @@ -1585,18 +1513,8 @@ static void as_merged_requests(request_q
>  	as_add_arq_hash(ad, arq);
>  
>  	if (rq_rb_key(req) != arq->rb_key) {
> -		struct as_rq *alias, *next_arq = NULL;
> -
> -		if (ad->next_arq[arq->is_sync] == arq)
> -			next_arq = as_find_next_arq(ad, arq);
> -
>  		as_del_arq_rb(ad, arq);
> -		if ((alias = as_add_arq_rb(ad, arq))) {
> -			list_del_init(&arq->fifo);
> -			as_add_aliased_request(ad, arq, alias);
> -			if (next_arq)
> -				ad->next_arq[arq->is_sync] = next_arq;
> -		}
> +		as_add_arq_rb(ad, arq);
>  	}
>  
>  	/*
> @@ -1616,18 +1534,6 @@ static void as_merged_requests(request_q
>  	}
>  
>  	/*
> -	 * Transfer list of aliases
> -	 */
> -	while (!list_empty(&next->queuelist)) {
> -		struct request *__rq = list_entry_rq(next->queuelist.next);
> -		struct as_rq *__arq = RQ_DATA(__rq);
> -
> -		list_move_tail(&__rq->queuelist, &req->queuelist);
> -
> -		WARN_ON(__arq->state != AS_RQ_QUEUED);
> -	}
> -
> -	/*
>  	 * kill knowledge of next, this one is a goner
>  	 */
>  	as_remove_queued_request(q, next);
> 
> 

-- 
Jens Axboe

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