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SubjectRe: [PATCH linux-2.6-block:master 01/05] blk: implement generic dispatch queue
 Hi, Jens.

On Thu, Oct 20, 2005 at 12:00:03PM +0200, Jens Axboe wrote:
> 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.

Maybe rename request_queue->last_sector too?

>
> > +/*
> > + * 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?
>

Taking backward seeking into account. I reasonsed that if specific
elevator chooses the next request with backward seeking,
elv_dispatch_insert() shouldn't change the order because that may
result in less efficient seek pattern. At the second thought,
specific elevators always perform sorting by itself in such cases, so
this seems unnecessary. I think we can strip this thing out.

> > + 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.

Fine.

>
> > 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.

Oh, please don't do that. Now, it's guaranteed that there are only
three paths a request can travel.

set_req_fn ->

i. add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn ->
(deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn
ii. add_req_fn -> (merged_fn ->)* -> merge_req_fn
iii. [none]

-> put_req_fn

These three are the only paths a request can travel. Also note that
dispatched requests don't get merged. So, after dispatched, the only
way out is via elevator_complete_req_fn and that's why that's the only
place ->boundary_rq is cleared. I've also documented above in biodoc
so that we can simplify codes knowing above information.

boundary_rq is used to keep request sorting sane when some pre-sorted
requests are present in the dispatch queue. Without it request
sorting acts wierdly when barrier requests are in the dispatch queue.

>
> > 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 :)

Somebody, please kill that thing.

Thanks. :-)

--
tejun
-
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