Re: Polled I/O cannot find completions

[Bijan Mottahedeh <bijan.mottahedeh@oracle.com>]

Does io_uring though have to deal with BLK_QC_T_NONE at all?  Or are you 
saying that it should never receive that result?
That's one of the things I'm not clear about.

--bijan

> CC'ing linux-block, this isn't an io_uring issue.
>
>
> On 3/26/20 8:57 PM, Bijan Mottahedeh wrote:
>> I'm seeing poll threads hang as I increase the number of threads in
>> polled fio tests.  I think this is because of polling on BLK_QC_T_NONE
>> cookie, which will never succeed.
>>
>> A related problem however, is that the meaning of BLK_QC_T_NONE seems to
>> be ambiguous.
>>
>> Specifically, the following cases return BLK_QC_T_NONE which I think
>> would be problematic for polled io:
>>
>>
>> generic_make_request()
>> ...
>>           if (current->bio_list) {
>>                   bio_list_add(&current->bio_list[0], bio);
>>                   goto out;
>>           }
>>
>> In this case the request is delayed but should get a cookie eventually.
>> How does the caller know what the right action is in this case for a
>> polled request?  Polling would never succeed.
>>
>>
>> __blk_mq_issue_directly()
>> ...
>>           case BLK_STS_RESOURCE:
>>           case BLK_STS_DEV_RESOURCE:
>>                   blk_mq_update_dispatch_busy(hctx, true);
>>                   __blk_mq_requeue_request(rq);
>>                   break;
>>
>> In this case, cookie is not updated and would keep its default
>> BLK_QC_T_NONE value from blk_mq_make_request().  However, this request
>> will eventually be reissued, so again, how would the caller poll for the
>> completion of this request?
>>
>> blk_mq_try_issue_directly()
>> ...
>>           ret = __blk_mq_try_issue_directly(hctx, rq, cookie, false, true);
>>           if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE)
>>                   blk_mq_request_bypass_insert(rq, false, true);
>>
>> Am I missing something here?
>>
>> Incidentally, I don't see BLK_QC_T_EAGAIN used anywhere, should it be?
>>
>> Thanks.
>>
>> --bijan
>>
>>
>>
>>
>