On 12/7/18 9:34 AM, Jens Axboe wrote:
> On 12/6/18 6:22 PM, jianchao.wang wrote:
>>
>>
>> On 12/7/18 9:13 AM, Jens Axboe wrote:
>>> On 12/6/18 6:04 PM, jianchao.wang wrote:
>>>>
>>>>
>>>> On 12/7/18 6:20 AM, Jens Axboe wrote:
>>>>> After the direct dispatch corruption fix, we permanently disallow direct
>>>>> dispatch of non read/write requests. This works fine off the normal IO
>>>>> path, as they will be retried like any other failed direct dispatch
>>>>> request. But for the blk_insert_cloned_request() that only DM uses to
>>>>> bypass the bottom level scheduler, we always first attempt direct
>>>>> dispatch. For some types of requests, that's now a permanent failure,
>>>>> and no amount of retrying will make that succeed.
>>>>>
>>>>> Don't use direct dispatch off the cloned insert path, always just use
>>>>> bypass inserts. This still bypasses the bottom level scheduler, which is
>>>>> what DM wants.
>>>>>
>>>>> Fixes: ffe81d45322c ("blk-mq: fix corruption with direct issue")
>>>>> Signed-off-by: Jens Axboe <ax...@kernel.dk>
>>>>>
>>>>> ---
>>>>>
>>>>> diff --git a/block/blk-core.c b/block/blk-core.c
>>>>> index deb56932f8c4..4c44e6fa0d08 100644
>>>>> --- a/block/blk-core.c
>>>>> +++ b/block/blk-core.c
>>>>> @@ -2637,7 +2637,8 @@ blk_status_t blk_insert_cloned_request(struct
>>>>> request_queue *q, struct request *
>>>>> * bypass a potential scheduler on the bottom device for
>>>>> * insert.
>>>>> */
>>>>> - return blk_mq_request_issue_directly(rq);
>>>>> + blk_mq_request_bypass_insert(rq, true);
>>>>> + return BLK_STS_OK;
>>>>> }
>>>>>
>>>>> spin_lock_irqsave(q->queue_lock, flags);
>>>>>
>>>> Not sure about this because it will break the merging promotion for
>>>> request based DM
>>>> from Ming.
>>>> 396eaf21ee17c476e8f66249fb1f4a39003d0ab4
>>>> (blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request
>>>> feedback)
>>>>
>>>> We could use some other way to fix this.
>>>
>>> That really shouldn't matter as this is the cloned insert, merging should
>>> have been done on the original request.
>>>
>>>
>> Just quote some comments from the patch.
>>
>> "
>> But dm-rq currently can't get the underlying queue's
>> dispatch feedback at all. Without knowing whether a request was issued
>> or not (e.g. due to underlying queue being busy) the dm-rq elevator will
>> not be able to provide effective IO merging (as a side-effect of dm-rq
>> currently blindly destaging a request from its elevator only to requeue
>> it after a delay, which kills any opportunity for merging). This
>> obviously causes very bad sequential IO performance.
>> ...
>> With this, request-based DM's blk-mq sequential IO performance is vastly
>> improved (as much as 3X in mpath/virtio-scsi testing)
>> "
>>
>> Using blk_mq_request_bypass_insert to replace the
>> blk_mq_request_issue_directly
>> could be a fast method to fix the current issue. Maybe we could get the
>> merging
>> promotion back after some time.
>
> This really sucks, mostly because DM wants to have it both ways - not use
> the bottom level IO scheduler, but still actually use it if it makes sense.
>
> There is another way to fix this - still do the direct dispatch, but have
> dm track if it failed and do bypass insert in that case. I didn't want do
> to that since it's more involved, but it's doable.
>
> Let me cook that up and test it... Don't like it, though.
>
Actually, I have tried to fix this issue in the 1st patch of my patchset
blk-mq: refactor code of issue directly.
Just insert the non-read-write command into dispatch list directly and return
BLK_STS_OK.
Thanks
Jianchao
