Re: [PATCH 1/1] virtio_blk: implement init_hctx MQ operation

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Aug 01, 2024 at 06:39:16PM +0300, Max Gurtovoy wrote:
> 
> On 01/08/2024 18:29, Michael S. Tsirkin wrote:
> > On Thu, Aug 01, 2024 at 06:17:21PM +0300, Max Gurtovoy wrote:
> > > On 01/08/2024 18:13, Michael S. Tsirkin wrote:
> > > > On Thu, Aug 01, 2024 at 06:11:37PM +0300, Max Gurtovoy wrote:
> > > > > In this operation set the driver data of the hctx to point to the virtio
> > > > > block queue. By doing so, we can use this reference in the and reduce
> > > > in the .... ?
> > > sorry for the type.
> > > 
> > > should be :
> > > 
> > > "By doing so, we can use this reference and reduce the number of operations in the fast path."
> > ok. what kind of benefit do you see with this patch?
> 
> As mentioned. This is a micro optimization that reduce the number of
> instructions/dereferences in the fast path.

By how much? How random code tweaks affect object code is unpredictable.
Pls show results of objdump to prove it does anything
useful.

> 
> > 
> > > > > the number of operations in the fast path.
> > > > > 
> > > > > Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> > > > > ---
> > > > >    drivers/block/virtio_blk.c | 42 ++++++++++++++++++++------------------
> > > > >    1 file changed, 22 insertions(+), 20 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > > > > index 2351f411fa46..35a7a586f6f5 100644
> > > > > --- a/drivers/block/virtio_blk.c
> > > > > +++ b/drivers/block/virtio_blk.c
> > > > > @@ -129,14 +129,6 @@ static inline blk_status_t virtblk_result(u8 status)
> > > > >    	}
> > > > >    }
> > > > > -static inline struct virtio_blk_vq *get_virtio_blk_vq(struct blk_mq_hw_ctx *hctx)
> > > > > -{
> > > > > -	struct virtio_blk *vblk = hctx->queue->queuedata;
> > > > > -	struct virtio_blk_vq *vq = &vblk->vqs[hctx->queue_num];
> > > > > -
> > > > > -	return vq;
> > > > > -}
> > > > > -
> > > > >    static int virtblk_add_req(struct virtqueue *vq, struct virtblk_req *vbr)
> > > > >    {
> > > > >    	struct scatterlist out_hdr, in_hdr, *sgs[3];
> > > > > @@ -377,8 +369,7 @@ static void virtblk_done(struct virtqueue *vq)
> > > > >    static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx)
> > > > >    {
> > > > > -	struct virtio_blk *vblk = hctx->queue->queuedata;
> > > > > -	struct virtio_blk_vq *vq = &vblk->vqs[hctx->queue_num];
> > > > > +	struct virtio_blk_vq *vq = hctx->driver_data;
> > > > >    	bool kick;
> > > > >    	spin_lock_irq(&vq->lock);
> > > > > @@ -428,10 +419,10 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
> > > > >    			   const struct blk_mq_queue_data *bd)
> > > > >    {
> > > > >    	struct virtio_blk *vblk = hctx->queue->queuedata;
> > > > > +	struct virtio_blk_vq *vq = hctx->driver_data;
> > > > >    	struct request *req = bd->rq;
> > > > >    	struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
> > > > >    	unsigned long flags;
> > > > > -	int qid = hctx->queue_num;
> > > > >    	bool notify = false;
> > > > >    	blk_status_t status;
> > > > >    	int err;
> > > > > @@ -440,26 +431,26 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
> > > > >    	if (unlikely(status))
> > > > >    		return status;
> > > > > -	spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
> > > > > -	err = virtblk_add_req(vblk->vqs[qid].vq, vbr);
> > > > > +	spin_lock_irqsave(&vq->lock, flags);
> > > > > +	err = virtblk_add_req(vq->vq, vbr);
> > > > >    	if (err) {
> > > > > -		virtqueue_kick(vblk->vqs[qid].vq);
> > > > > +		virtqueue_kick(vq->vq);
> > > > >    		/* Don't stop the queue if -ENOMEM: we may have failed to
> > > > >    		 * bounce the buffer due to global resource outage.
> > > > >    		 */
> > > > >    		if (err == -ENOSPC)
> > > > >    			blk_mq_stop_hw_queue(hctx);
> > > > > -		spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
> > > > > +		spin_unlock_irqrestore(&vq->lock, flags);
> > > > >    		virtblk_unmap_data(req, vbr);
> > > > >    		return virtblk_fail_to_queue(req, err);
> > > > >    	}
> > > > > -	if (bd->last && virtqueue_kick_prepare(vblk->vqs[qid].vq))
> > > > > +	if (bd->last && virtqueue_kick_prepare(vq->vq))
> > > > >    		notify = true;
> > > > > -	spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
> > > > > +	spin_unlock_irqrestore(&vq->lock, flags);
> > > > >    	if (notify)
> > > > > -		virtqueue_notify(vblk->vqs[qid].vq);
> > > > > +		virtqueue_notify(vq->vq);
> > > > >    	return BLK_STS_OK;
> > > > >    }
> > > > > @@ -504,7 +495,7 @@ static void virtio_queue_rqs(struct request **rqlist)
> > > > >    	struct request *requeue_list = NULL;
> > > > >    	rq_list_for_each_safe(rqlist, req, next) {
> > > > > -		struct virtio_blk_vq *vq = get_virtio_blk_vq(req->mq_hctx);
> > > > > +		struct virtio_blk_vq *vq = req->mq_hctx->driver_data;
> > > > >    		bool kick;
> > > > >    		if (!virtblk_prep_rq_batch(req)) {
> > > > > @@ -1164,6 +1155,16 @@ static const struct attribute_group *virtblk_attr_groups[] = {
> > > > >    	NULL,
> > > > >    };
> > > > > +static int virtblk_init_hctx(struct blk_mq_hw_ctx *hctx, void *data,
> > > > > +		unsigned int hctx_idx)
> > > > > +{
> > > > > +	struct virtio_blk *vblk = data;
> > > > > +	struct virtio_blk_vq *vq = &vblk->vqs[hctx_idx];
> > > > > +
> > > > > +	hctx->driver_data = vq;
> > > > > +	return 0;
> > > > > +}
> > > > > +
> > > > >    static void virtblk_map_queues(struct blk_mq_tag_set *set)
> > > > >    {
> > > > >    	struct virtio_blk *vblk = set->driver_data;
> > > > > @@ -1205,7 +1206,7 @@ static void virtblk_complete_batch(struct io_comp_batch *iob)
> > > > >    static int virtblk_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
> > > > >    {
> > > > >    	struct virtio_blk *vblk = hctx->queue->queuedata;
> > > > > -	struct virtio_blk_vq *vq = get_virtio_blk_vq(hctx);
> > > > > +	struct virtio_blk_vq *vq = hctx->driver_data;
> > > > >    	struct virtblk_req *vbr;
> > > > >    	unsigned long flags;
> > > > >    	unsigned int len;
> > > > > @@ -1236,6 +1237,7 @@ static const struct blk_mq_ops virtio_mq_ops = {
> > > > >    	.queue_rqs	= virtio_queue_rqs,
> > > > >    	.commit_rqs	= virtio_commit_rqs,
> > > > >    	.complete	= virtblk_request_done,
> > > > > +	.init_hctx	= virtblk_init_hctx,
> > > > >    	.map_queues	= virtblk_map_queues,
> > > > >    	.poll		= virtblk_poll,
> > > > >    };
> > > > > -- 
> > > > > 2.18.1