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

[Tejun Heo <htejun@gmail.com>]

Jens Axboe wrote:
> On Thu, Oct 20 2005, Tejun Heo wrote:
> 
>> 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?
> 
> 
> Yeah agree.
> 
> 
>>>>+/*
>>>>+ * 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.
> 
> 
> It wasn't so much the actual action as the logic. You overwrite boundary
> right away and it seems really strange to complare the absolute rq
> location with the max_back_in_sectors offset?
> 
> But lets just kill it, care to send a patch when I have pushed this
> stuff out?
> 

  Sure.

> 
>>>> 		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.
> 
> 
> Ah, it's my mistake, you only set it on dispatch. I was thinking it had
> an earlier life time, so there's no bug there at all. Thanks for
> clearing that up.
> 

  Thanks.

-- 
tejun