From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jens Axboe Subject: Re: [PATCH v1 5/9] block: loop: convert to blk-mq Date: Tue, 19 Aug 2014 15:50:37 -0500 Message-ID: <53F3B89D.6070703@kernel.dk> References: <1408031441-31156-1-git-send-email-ming.lei@canonical.com> <1408031441-31156-6-git-send-email-ming.lei@canonical.com> <20140815163111.GA16652@infradead.org> <53EE370D.1060106@kernel.dk> <53EE3966.60609@kernel.dk> <53F0EAEC.9040505@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Christoph Hellwig , Linux Kernel Mailing List , Andrew Morton , Dave Kleikamp , Zach Brown , Benjamin LaHaise , Kent Overstreet , open@kvack.org, list@kvack.org:AIO , Linux FS Devel , Dave Chinner To: Ming Lei Return-path: In-Reply-To: Sender: owner-linux-aio@kvack.org List-Id: linux-fsdevel.vger.kernel.org On 2014-08-18 06:53, Ming Lei wrote: > On Mon, Aug 18, 2014 at 9:22 AM, Ming Lei wrote: >> On Mon, Aug 18, 2014 at 1:48 AM, Jens Axboe wrote: >>> On 2014-08-16 02:06, Ming Lei wrote: >>>> >>>> On 8/16/14, Jens Axboe 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, 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. > I will keep the work queue in loop-mq V2, and it should be easy to switch > to the mechanism once it is ready. Reworked a bit more: http://git.kernel.dk/?p=linux-block.git;a=commit;h=a323185a761b9a54dc340d383695b4205ea258b6 Lets base loop-mq on the blk-mq workqueues, it would simplify it quite a bit and I don't think there's much point in doing v1 and then ripping it out for v2. Especially since it isn't queued up for 3.18 yet. -- Jens Axboe -- To unsubscribe, send a message with 'unsubscribe linux-aio' in the body to majordomo@kvack.org. For more info on Linux AIO, see: http://www.kvack.org/aio/ Don't email: aart@kvack.org