On Wed, Nov 28, 2018 at 08:13:43PM -0700, Jens Axboe wrote: > On 11/28/18 7:51 PM, Ming Lei wrote: > > On Wed, Nov 28, 2018 at 07:19:09PM -0700, Jens Axboe wrote: > >> On 11/28/18 6:23 PM, Ming Lei wrote: > >>> On Tue, Nov 27, 2018 at 07:34:51PM -0700, Jens Axboe wrote: > >>>> On 11/27/18 7:10 PM, Ming Lei wrote: > >>>>> On Mon, Nov 26, 2018 at 09:35:53AM -0700, Jens Axboe wrote: > >>>>>> We need this for blk-mq to kick things into gear, if we told it that > >>>>>> we had more IO coming, but then failed to deliver on that promise. > >>>>>> > >>>>>> Signed-off-by: Jens Axboe <[email protected]> > >>>>>> --- > >>>>>> drivers/block/virtio_blk.c | 15 +++++++++++++++ > >>>>>> 1 file changed, 15 insertions(+) > >>>>>> > >>>>>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c > >>>>>> index 6e869d05f91e..b49c57e77780 100644 > >>>>>> --- a/drivers/block/virtio_blk.c > >>>>>> +++ b/drivers/block/virtio_blk.c > >>>>>> @@ -214,6 +214,20 @@ static void virtblk_done(struct virtqueue *vq) > >>>>>> spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags); > >>>>>> } > >>>>>> > >>>>>> +static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx) > >>>>>> +{ > >>>>>> + struct virtio_blk *vblk = hctx->queue->queuedata; > >>>>>> + int qid = hctx->queue_num; > >>>>>> + bool kick; > >>>>>> + > >>>>>> + spin_lock_irq(&vblk->vqs[qid].lock); > >>>>>> + kick = virtqueue_kick_prepare(vblk->vqs[qid].vq); > >>>>>> + spin_unlock_irq(&vblk->vqs[qid].lock); > >>>>>> + > >>>>>> + if (kick) > >>>>>> + virtqueue_notify(vblk->vqs[qid].vq); > >>>>>> +} > >>>>>> + > >>>>>> static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx, > >>>>>> const struct blk_mq_queue_data *bd) > >>>>>> { > >>>>>> @@ -638,6 +652,7 @@ static void virtblk_initialize_rq(struct request > >>>>>> *req) > >>>>>> > >>>>>> static const struct blk_mq_ops virtio_mq_ops = { > >>>>>> .queue_rq = virtio_queue_rq, > >>>>>> + .commit_rqs = virtio_commit_rqs, > >>>>>> .complete = virtblk_request_done, > >>>>>> .init_request = virtblk_init_request, > >>>>>> #ifdef CONFIG_VIRTIO_BLK_SCSI > >>>>>> -- > >>>>>> 2.17.1 > >>>>>> > >>>>> > >>>>> If .commit_rqs() is implemented, virtqueue_notify() in virtio_queue_rq() > >>>>> should have been removed for saving the world switch per .queue_rq() > >>>> > >>>> ->commits_rqs() is only for the case where bd->last is set to false, > >>>> and we never make it to the end and flag bd->last == true. If bd->last > >>>> is true, the driver should kick things into gear. > >>> > >>> OK, looks I misunderstood it. However, virtio-blk doesn't need this > >>> change since virtio_queue_rq() can handle it well. This patch may > >>> introduce > >>> one unnecessary VM world switch in case of queue busy. > >> > >> Not it won't, it may in the case of some failure outside of the driver. > > > > If the failure is because of out of tag, blk_mq_dispatch_wake() will > > rerun the queue, and the bd->last will be set finally. Or is there > > other failure(outside of driver) not covered? > > The point is to make this happen when we commit the IOs, not needing to > do a restart (or relying on IO being in-flight). If we're submitting a > string of requests, we should not rely on failures happening only due to > IO being going and thus restarting us. It defeats the purpose of even > having ->last in the first place.
OK, it makes sense. > > >> The only reason that virtio-blk doesn't currently hang is because it > >> has restart logic, and the failure case only happens in the if we > >> already have IO in-flight. > > > > Yeah, virtqueue_kick() is called in case of any error in > > virtio_queue_rq(), so I am still wondering why we have to implement > > .commit_rqs() for virtio-blk. > > It's not strictly needed for virtio-blk with the restart logic that it > has, but I think it'd be nicer to kill that since we have other real use > cases of bd->last at this point. > > >>> IMO bd->last won't work well in case of io scheduler given the rq_list > >>> only includes one single request. > >> > >> But that's a fake limitation that definitely should just be lifted, > >> the fact that blk-mq-sched is _currently_ just doing single requests > >> is woefully inefficient. > > > > I agree, but seems a bit hard given we have to consider request > > merge. > > We don't have to drain everything, it should still be feasible to submit > at least a batch of requests. For basic sequential IO, you want to leave > the last one in the queue, if you have IOs going, for instance. But > doing each and every request individually is a huge extra task. Doing > IOPS comparisons of kyber and no scheduler reveals that to be very true. > > >>> I wrote this kind of patch(never posted) before to use sort of > >>> ->commits_rqs() to replace the current bd->last mechanism which need > >>> one extra driver tag, which may improve the above case, also code gets > >>> cleaned up. > >> > >> It doesn't need one extra driver tag, we currently get an extra one just > >> to flag ->last correctly. That's not a requirement, that's a limitation > >> of the current implementation. We could get rid of that, and it it > >> proves to be an issue, that's not hard to do. > > > > What do you think about using .commit_rqs() to replace ->last? For > > example, just call .commit_rqs() after the last request is queued to > > driver successfully. Then we can remove bd->last and avoid to get the > > extra tag for figuring out bd->last. > > I don't want to make ->commit_rqs() part of the regular execution, it > should be relegated to the "failure" case of not being able to fulfil > our promise of sending a request with bd->last == true. Reasons > mentioned earlier, but basically it's more efficient to commit from > inside ->queue_rq() if we can, so we don't have to re-grab the > submission lock needlessly. > > I like the idea of separate ->queue and ->commit, but in practice I > don't see it working out without a performance penalty. Thanks for your detailed explanation, this patch looks fine: Reviewed-by: Ming Lei <[email protected]> Thanks, Ming
