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

[Ming Lei]

On Thu, Aug 21, 2014 at 12:09 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> On 2014-08-19 20:23, Ming Lei wrote:
>>
>> On Wed, Aug 20, 2014 at 4:50 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
>>>
>>> On 2014-08-18 06:53, Ming Lei wrote:
>>>>
>>>>
>>>> On Mon, Aug 18, 2014 at 9:22 AM, Ming Lei <ming.lei@xxxxxxxxxxxxx>
>>>> wrote:
>>>>>
>>>>>
>>>>> On Mon, Aug 18, 2014 at 1:48 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
>>>>>>
>>>>>>
>>>>>> On 2014-08-16 02:06, Ming Lei wrote:
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> On 8/16/14, Jens Axboe <axboe@xxxxxxxxx> 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.
>>>
>>>
>>>
>>> It's insane, but it can happen. And given how cheap it is to do a
>>> workqueue,
>>
>>
>> Workqueue with WQ_MEM_RECLAIM need to create a standalone kthread
>> for the queue, so at default there will be 8 kthreads created even no one
>> uses loop at all.  From current implementation the per-device thread is
>> created only when one file or blk device is attached to the loop device,
>> which
>> may not be possible when blk-mq supports per-device workqueue.
>
>
> That is true, but I don't see this as a huge problem. And idle kthread is
> pretty much free...

OK, I am fine with that too if no one complains that, :-)

BTW, loop over loop won't be a problem since loop driver can cut the
dependency and just use the original back file, so one workqueue should
be enough for all loop devices.

>
>
>>> I don't see a reason why we should not. Loop over loop might seem nutty,
>>> but
>>> it's not that far out into the realm of nutty things that people end up
>>> doing.
>>
>>
>> Another reason I am still not sure if workqueue is good for loop, though I
>> do really like workqueue for sake of simplicity, :-)
>>
>> - sequential read becomes a bit slow with workqueue, especially for some
>> fast block(such as null_blk)
>>
>> - random read becomes a bit slow too for some fast devices(such as
>> null_blk)
>> in some environment(It is reproduced in my server, but can't in my laptop)
>> even
>> it can improve throughout quite much for common devices(HDD., SSD,..)
>
>
> Thread offloading will always slow down some use cases, like sync(ish) IO.
> Not sure this is a case against kthread vs workqueue, performance and
> behavior should be identical here?

Looks no sync is involved because I just test randread with fio, and
the cause should be same with below.

>
>
>>  From my investigation, context switch increases almost 50% with
>> workqueue compared with kthread in loop in a quad-core VM. With
>> kthread, requests may be handled as batch in cases which won't be
>> blocked in read()/write()(like null_blk, tmpfs, ...), but it is impossible
>> with
>> workqueue any more.  Also block plug&unplug should have been used
>> with kthread to optimize the case, especially when kernel AIO is applied,
>> still impossible with work queue too.
>
>
> OK, that one is actually a good point, since one need not do per-item
> queueing. We could handle different units, though. And we should have proper
> marking of the last item in a chain of stuff, so we might even be able to
> offload based on that instead of doing single items. It wont help the sync
> case, but for that, workqueue and kthread would be identical.

We may do that by introducing callback of queue_rq_list in blk_mq_ops,
and I will figure out one patch today to see if it can help the case.

> Or we could just provide a better alternative in blk-mq. Doing workqueues is
> just so damn easy, I'd be reluctant to add a kthread pool instead. It'd be
> much better to augment or fix workqueues to work well for this case as well.



Thanks,
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