On Mon, Aug 18, 2014 at 9:22 AM, Ming Lei <ming.lei@xxxxxxxxxxxxx> wrote: > On Mon, Aug 18, 2014 at 1:48 AM, Jens Axboe <axboe@xxxxxxxxx> wrote: >> 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. > > 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. I will keep the work queue in loop-mq V2, and it should be easy to switch to the mechanism once it is ready. Thanks, -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
