[dm-devel] [RFC PATCH V2 09/13] block: use per-task poll context to implement bio based io poll

[Sagi Grimberg]

>>> +static void blk_bio_poll_post_submit(struct bio *bio, blk_qc_t cookie)
>>> +{
>>> +	bio->bi_iter.bi_private_data = cookie;
>>> +}
>>> +
>>
>> Hey Ming, thinking about nvme-mpath, I'm thinking that this should be
>> an exported function for failover. nvme-mpath updates bio.bi_dev
>> when re-submitting I/Os to an alternate path, so I'm thinking
>> that if this function is exported then nvme-mpath could do as little
>> as the below to allow polling?
>>
>> --
>> diff --git a/drivers/nvme/host/multipath.c b/drivers/nvme/host/multipath.c
>> index 92adebfaf86f..e562e296153b 100644
>> --- a/drivers/nvme/host/multipath.c
>> +++ b/drivers/nvme/host/multipath.c
>> @@ -345,6 +345,7 @@ static void nvme_requeue_work(struct work_struct *work)
>>          struct nvme_ns_head *head =
>>                  container_of(work, struct nvme_ns_head, requeue_work);
>>          struct bio *bio, *next;
>> +       blk_qc_t cookie;
>>
>>          spin_lock_irq(&head->requeue_lock);
>>          next = bio_list_get(&head->requeue_list);
>> @@ -359,7 +360,8 @@ static void nvme_requeue_work(struct work_struct *work)
>>                   * path.
>>                   */
>>                  bio_set_dev(bio, head->disk->part0);
>> -               submit_bio_noacct(bio);
>> +               cookie = submit_bio_noacct(bio);
>> +               blk_bio_poll_post_submit(bio, cookie);
>>          }
>>   }
>> --
>>
>> I/O failover will create misalignment from the polling context cpu and
>> the submission cpu (running requeue_work), but I don't see if there is
>> something that would break here...
> 
> I understand requeue shouldn't be one usual event, and I guess it is just
> fine to fallback to IRQ based mode?

Well, when it will failover, it will probably be directed to the poll
queues. Maybe I'm missing something...

> This patchset actually doesn't cover such bio submission from kernel context.

What is the difference?