Re: [PATCH v1 5/9] block: loop: convert to blk-mq
From: Jens Axboe
Date: Sun Aug 17 2014 - 13:48:37 EST
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.
--
Jens Axboe
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