Re: [PATCH 5/7] blk-mq-sched: add framework for MQ capable IO schedulers

From: Bart Van Assche
Date: Wed Dec 14 2016 - 05:32:08 EST

On 12/13/2016 04:14 PM, Jens Axboe wrote:
> On 12/13/2016 06:56 AM, Bart Van Assche wrote:
>> On 12/08/2016 09:13 PM, Jens Axboe wrote:
>>> +struct request *blk_mq_sched_alloc_shadow_request(struct request_queue *q,
>>> + struct blk_mq_alloc_data *data,
>>> + struct blk_mq_tags *tags,
>>> + atomic_t *wait_index)
>>> +{
>> Using the word "shadow" in the function name suggests to me that there
>> is a shadow request for every request and a request for every shadow
>> request. However, my understanding from the code is that there can be
>> requests without shadow requests (for e.g. a flush) and shadow requests
>> without requests. Shouldn't the name of this function reflect that, e.g.
>> by using "sched" or "elv" in the function name instead of "shadow"?
> Shadow might not be the best name. Most do have shadows though, it's
> only the rare exception like the flush, that you mention. I'll see if I
> can come up with a better name.

Hello Jens,

One aspect of this patch series that might turn out to be a maintenance
burden is the copying between original and shadow requests. It is easy
to overlook that rq_copy() has to be updated if a field would ever be
added to struct request. Additionally, having to allocate two requests
structures per I/O instead of one will have a runtime overhead. Do you
think the following approach would work?
- Instead of using two request structures per I/O, only use a single
request structure.
- Instead of storing one tag in the request structure, store two tags
in that structure. One tag comes from the I/O scheduler tag set
(size: nr_requests) and the other from the tag set associated with
the block driver (size: HBA queue depth).
- Only add a request to the hctx dispatch list after a block driver tag
has been assigned. This means that an I/O scheduler must keep a
request structure on a list it manages itself as long as no block
driver tag has been assigned.
- sysfs_list_show() is modified such that it shows both tags.