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SubjectRe: [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@redhat.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

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