linux-kernel - Re: [PATCH v4 2/5] vsock/virtio: reduce credit update messages

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Date:   Tue, 3 Sep 2019 09:31:20 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        virtualization@...ts.linux-foundation.org,
        Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v4 2/5] vsock/virtio: reduce credit update messages

On Tue, Sep 03, 2019 at 12:38:02AM -0400, Michael S. Tsirkin wrote:
> On Wed, Jul 17, 2019 at 01:30:27PM +0200, Stefano Garzarella wrote:
> > In order to reduce the number of credit update messages,
> > we send them only when the space available seen by the
> > transmitter is less than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE.
> > 
> > Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>
> > ---
> >  include/linux/virtio_vsock.h            |  1 +
> >  net/vmw_vsock/virtio_transport_common.c | 16 +++++++++++++---
> >  2 files changed, 14 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> > index 7d973903f52e..49fc9d20bc43 100644
> > --- a/include/linux/virtio_vsock.h
> > +++ b/include/linux/virtio_vsock.h
> > @@ -41,6 +41,7 @@ struct virtio_vsock_sock {
> >  
> >  	/* Protected by rx_lock */
> >  	u32 fwd_cnt;
> > +	u32 last_fwd_cnt;
> >  	u32 rx_bytes;
> >  	struct list_head rx_queue;
> >  };
> > diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> > index 095221f94786..a85559d4d974 100644
> > --- a/net/vmw_vsock/virtio_transport_common.c
> > +++ b/net/vmw_vsock/virtio_transport_common.c
> > @@ -211,6 +211,7 @@ static void virtio_transport_dec_rx_pkt(struct virtio_vsock_sock *vvs,
> >  void virtio_transport_inc_tx_pkt(struct virtio_vsock_sock *vvs, struct virtio_vsock_pkt *pkt)
> >  {
> >  	spin_lock_bh(&vvs->tx_lock);
> > +	vvs->last_fwd_cnt = vvs->fwd_cnt;
> >  	pkt->hdr.fwd_cnt = cpu_to_le32(vvs->fwd_cnt);
> >  	pkt->hdr.buf_alloc = cpu_to_le32(vvs->buf_alloc);
> >  	spin_unlock_bh(&vvs->tx_lock);
> > @@ -261,6 +262,7 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> >  	struct virtio_vsock_sock *vvs = vsk->trans;
> >  	struct virtio_vsock_pkt *pkt;
> >  	size_t bytes, total = 0;
> > +	u32 free_space;
> >  	int err = -EFAULT;
> >  
> >  	spin_lock_bh(&vvs->rx_lock);
> > @@ -291,11 +293,19 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> >  			virtio_transport_free_pkt(pkt);
> >  		}
> >  	}
> > +
> > +	free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
> > +
> >  	spin_unlock_bh(&vvs->rx_lock);
> >  
> > -	/* Send a credit pkt to peer */
> > -	virtio_transport_send_credit_update(vsk, VIRTIO_VSOCK_TYPE_STREAM,
> > -					    NULL);
> > +	/* We send a credit update only when the space available seen
> > +	 * by the transmitter is less than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE
> 
> This is just repeating what code does though.
> Please include the *reason* for the condition.
> E.g. here's a better comment:
> 
> 	/* To reduce number of credit update messages,
> 	 * don't update credits as long as lots of space is available.
> 	 * Note: the limit chosen here is arbitrary. Setting the limit
> 	 * too high causes extra messages. Too low causes transmitter
> 	 * stalls. As stalls are in theory more expensive than extra
> 	 * messages, we set the limit to a high value. TODO: experiment
> 	 * with different values.
> 	 */
> 

Yes, it is better, sorry for that. I'll try to avoid unnecessary comments,
explaining the reason for certain changes.

Since this patch is already queued in net-next, should I send another
patch to fix the comment?

Thanks,
Stefano