Re: [PATCH linux-2.6-block:master 01/05] blk: implement generic dispatch queue

[Jens Axboe <axboe@suse.de>]

On Wed, Oct 19 2005, Tejun Heo wrote:
> @@ -40,6 +40,11 @@
>  static DEFINE_SPINLOCK(elv_list_lock);
>  static LIST_HEAD(elv_list);
>  
> +static inline sector_t rq_last_sector(struct request *rq)
> +{
> +	return rq->sector + rq->nr_sectors;
> +}

Slightly misnamed, since it's really the sector after the last sector
:-)

I've renamed that to rq_end_sector() instead.

> +/*
> + * Insert rq into dispatch queue of q.  Queue lock must be held on
> + * entry.  If sort != 0, rq is sort-inserted; otherwise, rq will be
> + * appended to the dispatch queue.  To be used by specific elevators.
> + */
> +void elv_dispatch_insert(request_queue_t *q, struct request *rq, int sort)
> +{
> +	sector_t boundary;
> +	unsigned max_back;
> +	struct list_head *entry;
> +
> +	if (!sort) {
> +		/* Specific elevator is performing sort.  Step away. */
> +		q->last_sector = rq_last_sector(rq);
> +		q->boundary_rq = rq;
> +		list_add_tail(&rq->queuelist, &q->queue_head);
> +		return;
> +	}
> +
> +	boundary = q->last_sector;
> +	max_back = q->max_back_kb * 2;
> +	boundary = boundary > max_back ? boundary - max_back : 0;

This looks really strange, what are you doing with boundary here?

> +	list_for_each_prev(entry, &q->queue_head) {
> +		struct request *pos = list_entry_rq(entry);
> +
> +		if (pos->flags & (REQ_SOFTBARRIER|REQ_HARDBARRIER|REQ_STARTED))
> +			break;
> +		if (rq->sector >= boundary) {
> +			if (pos->sector < boundary)
> +				continue;
> +		} else {
> +			if (pos->sector >= boundary)
> +				break;
> +		}
> +		if (rq->sector >= pos->sector)
> +			break;
> +	}
> +
> +	list_add(&rq->queuelist, entry);
> +}

I've split this into, I don't like rolled-up functions that really do
two seperate things. So elv_dispatch_sort() now does sorting,
elv_dispatch_add_tail() does what !sort would have done.

>  	while ((rq = __elv_next_request(q)) != NULL) {
> -		/*
> -		 * just mark as started even if we don't start it, a request
> -		 * that has been delayed should not be passed by new incoming
> -		 * requests
> -		 */
> -		rq->flags |= REQ_STARTED;
> +		if (!(rq->flags & REQ_STARTED)) {
> +			elevator_t *e = q->elevator;
> +
> +			/*
> +			 * This is the first time the device driver
> +			 * sees this request (possibly after
> +			 * requeueing).  Notify IO scheduler.
> +			 */
> +			if (blk_sorted_rq(rq) &&
> +			    e->ops->elevator_activate_req_fn)
> +				e->ops->elevator_activate_req_fn(q, rq);
> +
> +			/*
> +			 * just mark as started even if we don't start
> +			 * it, a request that has been delayed should
> +			 * not be passed by new incoming requests
> +			 */
> +			rq->flags |= REQ_STARTED;
> +		}
>  
>  		if (rq == q->last_merge)
>  			q->last_merge = NULL;
>  
> +		if (!q->boundary_rq || q->boundary_rq == rq) {
> +			q->last_sector = rq_last_sector(rq);
> +			q->boundary_rq = NULL;
> +		}

This seems to be the only place where you clear ->boundary_rq, that
can't be right. What about rq-to-rq merging, ->boundary_rq could be
freed and you wont notice. Generally I don't really like keeping
pointers to rqs around, it's given us problems in the past with the
last_merge bits even. For now I've added a clear of this in
__blk_put_request() as well.

>  int elv_queue_empty(request_queue_t *q)
>  {
>  	elevator_t *e = q->elevator;
>  
> +	if (!list_empty(&q->queue_head))
> +		return 0;
> +
>  	if (e->ops->elevator_queue_empty_fn)
>  		return e->ops->elevator_queue_empty_fn(q);
>  
> -	return list_empty(&q->queue_head);
> +	return 1;
>  }

Agree, this order definitely makes more sense.

> @@ -2475,14 +2478,14 @@ static void __blk_put_request(request_qu
>  
>  void blk_put_request(struct request *req)
>  {
> +	unsigned long flags;
> +	request_queue_t *q = req->q;
> +
>  	/*
> -	 * if req->rl isn't set, this request didnt originate from the
> -	 * block layer, so it's safe to just disregard it
> +	 * Gee, IDE calls in w/ NULL q.  Fix IDE and remove the
> +	 * following if (q) test.
>  	 */
> -	if (req->rl) {
> -		unsigned long flags;
> -		request_queue_t *q = req->q;
> -
> +	if (q) {

The q == NULL is because ide is using requests allocated on the stack,
I've wanted for that to die for many years :)

-- 
Jens Axboe