Re: [PATCH v1 5/9] block: loop: convert to blk-mq

[Ming Lei <ming.lei@canonical.com>]

On Mon, Aug 18, 2014 at 9:22 AM, Ming Lei <ming.lei@canonical.com> wrote:
> On Mon, Aug 18, 2014 at 1:48 AM, Jens Axboe <axboe@kernel.dk> wrote:
>> On 2014-08-16 02:06, Ming Lei wrote:
>>>
>>> On 8/16/14, Jens Axboe <axboe@kernel.dk> wrote:
>>>>
>>>> On 08/15/2014 10:36 AM, Jens Axboe wrote:
>>>>>
>>>>> On 08/15/2014 10:31 AM, Christoph Hellwig wrote:
>>>>>>>
>>>>>>> +static void loop_queue_work(struct work_struct *work)
>>>>>>
>>>>>>
>>>>>> Offloading work straight to a workqueue dosn't make much sense
>>>>>> in the blk-mq model as we'll usually be called from one.  If you
>>>>>> need to avoid the cases where we are called directly a flag for
>>>>>> the blk-mq code to always schedule a workqueue sounds like a much
>>>>>> better plan.
>>>>>
>>>>>
>>>>> That's a good point - would clean up this bit, and be pretty close to a
>>>>> one-liner to support in blk-mq for the drivers that always need blocking
>>>>> context.
>>>>
>>>>
>>>> Something like this should do the trick - totally untested. But with
>>>> that, loop would just need to add BLK_MQ_F_WQ_CONTEXT to it's tag set
>>>> flags and it could always do the work inline from ->queue_rq().
>>>
>>>
>>> I think it is a good idea.
>>>
>>> But for loop, there may be two problems:
>>>
>>> - default max_active for bound workqueue is 256, which means several slow
>>> loop devices might slow down whole block system. With kernel AIO, it won't
>>> be a big deal, but some block/fs may not support direct I/O and still
>>> fallback to
>>> workqueue
>>>
>>> - 6. Guidelines of Documentation/workqueue.txt
>>> If there is dependency among multiple work items used during memory
>>> reclaim, they should be queued to separate wq each with WQ_MEM_RECLAIM.
>>
>>
>> Both are good points. But I think this mainly means that we should support
>> this through a potentially per-dispatch queue workqueue, separate from
>> kblockd. There's no reason blk-mq can't support this with a per-hctx
>> workqueue, for drivers that need it.
>
> Good idea, and per-device workqueue should be enough if
> BLK_MQ_F_WQ_CONTEXT flag is set.

Maybe for most of cases per-device class(driver) workqueue should be
enough since dependency between devices driven by same driver
isn't common, for example, loop over loop is absolutely insane.

I will keep the work queue in loop-mq V2, and it should be easy to switch
to the mechanism once it is ready.

Thanks,