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

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Message-ID: <53F0EAEC.9040505@kernel.dk>
Date:	Sun, 17 Aug 2014 11:48:28 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Ming Lei <ming.lei@...onical.com>
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 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.

-- 
Jens Axboe

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