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

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Date:	Sat, 16 Aug 2014 16:06:28 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Christoph Hellwig <hch@...radead.org>,
	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>, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v1 5/9] block: loop: convert to blk-mq

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.


Thanks,
-- 
Ming Lei
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