From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5A976C43441 for ; Thu, 29 Nov 2018 03:27:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1D7D5206B6 for ; Thu, 29 Nov 2018 03:27:32 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1D7D5206B6 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-block-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727098AbeK2ObX (ORCPT ); Thu, 29 Nov 2018 09:31:23 -0500 Received: from mx1.redhat.com ([209.132.183.28]:45802 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727026AbeK2ObX (ORCPT ); Thu, 29 Nov 2018 09:31:23 -0500 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 8AAE9369CA; Thu, 29 Nov 2018 03:27:30 +0000 (UTC) Received: from ming.t460p (ovpn-8-23.pek2.redhat.com [10.72.8.23]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 18D4660CCF; Thu, 29 Nov 2018 03:27:25 +0000 (UTC) Date: Thu, 29 Nov 2018 11:27:21 +0800 From: Ming Lei To: Jens Axboe Cc: linux-block@vger.kernel.org, linux-nvme@lists.infradead.org Subject: Re: [PATCH 5/8] virtio_blk: implement mq_ops->commit_rqs() hook Message-ID: <20181129032720.GD23390@ming.t460p> References: <20181126163556.5181-1-axboe@kernel.dk> <20181126163556.5181-6-axboe@kernel.dk> <20181128021029.GF11128@ming.t460p> <35b33a34-9e24-5acb-7a4e-57433328bf3d@kernel.dk> <20181129012342.GB23249@ming.t460p> <20181129025143.GC23390@ming.t460p> <41d9d590-6a59-c050-c8a8-2506342b93a4@kernel.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <41d9d590-6a59-c050-c8a8-2506342b93a4@kernel.dk> User-Agent: Mutt/1.9.1 (2017-09-22) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.30]); Thu, 29 Nov 2018 03:27:30 +0000 (UTC) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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 > >>>>>> --- > >>>>>> 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 Thanks, Ming