Re: [PATCH v3 2/2] virtio-blk: support mq_ops->queue_rqs()

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Mar 29, 2022 at 12:50:33AM +0900, Suwan Kim wrote:
> On Mon, Mar 28, 2022 at 02:16:13PM +0100, Stefan Hajnoczi wrote:
> > On Thu, Mar 24, 2022 at 11:04:50PM +0900, Suwan Kim wrote:
> > > +static void virtio_queue_rqs(struct request **rqlist)
> > > +{
> > > +	struct request *req, *next, *prev = NULL;
> > > +	struct request *requeue_list = NULL;
> > > +
> > > +	rq_list_for_each_safe(rqlist, req, next) {
> > > +		struct virtio_blk_vq *vq = req->mq_hctx->driver_data;
> > > +		unsigned long flags;
> > > +		bool kick;
> > > +
> > > +		if (!virtblk_prep_rq_batch(vq, req)) {
> > > +			rq_list_move(rqlist, &requeue_list, req, prev);
> > > +			req = prev;
> > > +
> > > +			if (!req)
> > > +				continue;
> > > +		}
> > > +
> > > +		if (!next || req->mq_hctx != next->mq_hctx) {
> > > +			spin_lock_irqsave(&vq->lock, flags);
> > 
> > Did you try calling virtblk_add_req() here to avoid acquiring and
> > releasing the lock multiple times? In other words, do virtblk_prep_rq()
> > but wait until we get here to do virtblk_add_req().
> > 
> > I don't know if it has any measurable effect on performance or maybe the
> > code would become too complex, but I noticed that we're not fully
> > exploiting batching.
> 
> I tried as you said. I called virtlblk_add_req() and added requests
> of rqlist to virtqueue in this if statement with holding the lock
> only once.
> 
> I attach the code at the end of this mail.
> Please refer the code.
> 
> But I didn't see improvement. It showed slightly worse performance
> than the current patch.

Okay, thanks for trying it!

> > > +			kick = virtqueue_kick_prepare(vq->vq);
> > > +			spin_unlock_irqrestore(&vq->lock, flags);
> > > +			if (kick)
> > > +				virtqueue_notify(vq->vq);
> > > +
> > > +			req->rq_next = NULL;
> 
> Did you ask this part?
> 
> > > +			*rqlist = next;
> > > +			prev = NULL;
> > > +		} else
> > > +			prev = req;
> > 
> > What guarantees that req is still alive after we called
> > virtblk_add_req()? The device may have seen it and completed it already
> > by the time we get here.
> 
> Isn't request completed after the kick?
> 
> If you asked about "req->rq_next = NULL",
> I think it should be placed before
> "kick = virtqueue_kick_prepare(vq->vq);"
> 
> -----------
> 	req->rq_next = NULL;
> 	kick = virtqueue_kick_prepare(vq->vq);
> 	spin_unlock_irqrestore(&vq->lock, flags);
> 	if (kick)
> 		virtqueue_notify(vq->vq);
> -----------

No, virtqueue_add_sgs() exposes vring descriptors to the device. The
device may process immediately. In other words, VIRTIO devices may poll
the vring instead of waiting for virtqueue_notify(). There is no
guarantee that the request is alive until virtqueue_notify() is called.

The code has to handle the case where the request is completed during
virtqueue_add_sgs().

Stefan

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