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

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Message-ID: <CACVXFVO3CrD2CoHiBOZ8GNMXrT2pJ=t2BzmAHksqRTmgrpsaaQ@mail.gmail.com>
Date:	Mon, 18 Aug 2014 09:22:10 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Kleikamp <dave.kleikamp@...cle.com>,
	Zach Brown <zab@...bo.net>, Benjamin LaHaise <bcrl@...ck.org>,
	Kent Overstreet <kmo@...erainc.com>,
	"open list:AIO" <linux-aio@...ck.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v1 5/9] block: loop: convert to blk-mq

On Mon, Aug 18, 2014 at 1:48 AM, Jens Axboe <axboe@...nel.dk> wrote:
> On 2014-08-16 02:06, Ming Lei wrote:
>>
>> On 8/16/14, Jens Axboe <axboe@...nel.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.

Thanks,
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