Subject: Re: [PATCH net-next v5 2/3] virtio/vsock: send credit update during setting SO_RCVLOWAT

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 30 Nov 2023 12:40:43 -0500

On Thu, Nov 30, 2023 at 03:11:19PM +0100, Stefano Garzarella wrote:
> On Thu, Nov 30, 2023 at 08:58:58AM -0500, Michael S. Tsirkin wrote:
> > On Thu, Nov 30, 2023 at 04:43:34PM +0300, Arseniy Krasnov wrote:
> > > 
> > > 
> > > On 30.11.2023 16:42, Michael S. Tsirkin wrote:
> > > > On Thu, Nov 30, 2023 at 04:08:39PM +0300, Arseniy Krasnov wrote:
> > > >> Send credit update message when SO_RCVLOWAT is updated and it is bigger
> > > >> than number of bytes in rx queue. It is needed, because 'poll()' will
> > > >> wait until number of bytes in rx queue will be not smaller than
> > > >> SO_RCVLOWAT, so kick sender to send more data. Otherwise mutual hungup
> > > >> for tx/rx is possible: sender waits for free space and receiver is
> > > >> waiting data in 'poll()'.
> > > >>
> > > >> Signed-off-by: Arseniy Krasnov <[email-masked]>
> > > >> ---
> > > >>  Changelog:
> > > >>  v1 -> v2:
> > > >>   * Update commit message by removing 'This patch adds XXX' manner.
> > > >>   * Do not initialize 'send_update' variable - set it directly during
> > > >>     first usage.
> > > >>  v3 -> v4:
> > > >>   * Fit comment in 'virtio_transport_notify_set_rcvlowat()' to 80 chars.
> > > >>  v4 -> v5:
> > > >>   * Do not change callbacks order in transport structures.
> > > >>
> > > >>  drivers/vhost/vsock.c                   |  1 +
> > > >>  include/linux/virtio_vsock.h            |  1 +
> > > >>  net/vmw_vsock/virtio_transport.c        |  1 +
> > > >>  net/vmw_vsock/virtio_transport_common.c | 27 +++++++++++++++++++++++++
> > > >>  net/vmw_vsock/vsock_loopback.c          |  1 +
> > > >>  5 files changed, 31 insertions(+)
> > > >>
> > > >> diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> > > >> index f75731396b7e..4146f80db8ac 100644
> > > >> --- a/drivers/vhost/vsock.c
> > > >> +++ b/drivers/vhost/vsock.c
> > > >> @@ -451,6 +451,7 @@ static struct virtio_transport vhost_transport = {
> > > >>  		.notify_buffer_size       = virtio_transport_notify_buffer_size,
> > > >>
> > > >>  		.read_skb = virtio_transport_read_skb,
> > > >> +		.notify_set_rcvlowat      = virtio_transport_notify_set_rcvlowat
> > > >>  	},
> > > >>
> > > >>  	.send_pkt = vhost_transport_send_pkt,
> > > >> diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
> > > >> index ebb3ce63d64d..c82089dee0c8 100644
> > > >> --- a/include/linux/virtio_vsock.h
> > > >> +++ b/include/linux/virtio_vsock.h
> > > >> @@ -256,4 +256,5 @@ void virtio_transport_put_credit(struct virtio_vsock_sock *vvs, u32 credit);
> > > >>  void virtio_transport_deliver_tap_pkt(struct sk_buff *skb);
> > > >>  int virtio_transport_purge_skbs(void *vsk, struct sk_buff_head *list);
> > > >>  int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t read_actor);
> > > >> +int virtio_transport_notify_set_rcvlowat(struct vsock_sock *vsk, int val);
> > > >>  #endif /* _LINUX_VIRTIO_VSOCK_H */
> > > >> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> > > >> index af5bab1acee1..8007593a3a93 100644
> > > >> --- a/net/vmw_vsock/virtio_transport.c
> > > >> +++ b/net/vmw_vsock/virtio_transport.c
> > > >> @@ -539,6 +539,7 @@ static struct virtio_transport virtio_transport = {
> > > >>  		.notify_buffer_size       = virtio_transport_notify_buffer_size,
> > > >>
> > > >>  		.read_skb = virtio_transport_read_skb,
> > > >> +		.notify_set_rcvlowat      = virtio_transport_notify_set_rcvlowat
> > > >>  	},
> > > >>
> > > >>  	.send_pkt = virtio_transport_send_pkt,
> > > >> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> > > >> index f6dc896bf44c..1cb556ad4597 100644
> > > >> --- a/net/vmw_vsock/virtio_transport_common.c
> > > >> +++ b/net/vmw_vsock/virtio_transport_common.c
> > > >> @@ -1684,6 +1684,33 @@ int virtio_transport_read_skb(struct vsock_sock *vsk, skb_read_actor_t recv_acto
> > > >>  }
> > > >>  EXPORT_SYMBOL_GPL(virtio_transport_read_skb);
> > > >>
> > > >> +int virtio_transport_notify_set_rcvlowat(struct vsock_sock *vsk,
> > > >> int val)
> > > >> +{
> > > >> +	struct virtio_vsock_sock *vvs = vsk->trans;
> > > >> +	bool send_update;
> > > >> +
> > > >> +	spin_lock_bh(&vvs->rx_lock);
> > > >> +
> > > >> +	/* If number of available bytes is less than new SO_RCVLOWAT value,
> > > >> +	 * kick sender to send more data, because sender may sleep in
> > > >> its
> > > >> +	 * 'send()' syscall waiting for enough space at our side.
> > > >> +	 */
> > > >> +	send_update = vvs->rx_bytes < val;
> > > >> +
> > > >> +	spin_unlock_bh(&vvs->rx_lock);
> > > >> +
> > > >> +	if (send_update) {
> > > >> +		int err;
> > > >> +
> > > >> +		err = virtio_transport_send_credit_update(vsk);
> > > >> +		if (err < 0)
> > > >> +			return err;
> > > >> +	}
> > > >> +
> > > >> +	return 0;
> > > >> +}
> > > >
> > > >
> > > > I find it strange that this will send a credit update
> > > > even if nothing changed since this was called previously.
> > > > I'm not sure whether this is a problem protocol-wise,
> > > > but it certainly was not envisioned when the protocol was
> > > > built. WDYT?
> > > 
> > > >From virtio spec I found:
> > > 
> > > It is also valid to send a VIRTIO_VSOCK_OP_CREDIT_UPDATE packet without previously receiving a
> > > VIRTIO_VSOCK_OP_CREDIT_REQUEST packet. This allows communicating updates any time a change
> > > in buffer space occurs.
> > > So I guess there is no limitations to send such type of packet, e.g. it is not
> > > required to be a reply for some another packet. Please, correct me if im wrong.
> > > 
> > > Thanks, Arseniy
> > 
> > 
> > Absolutely. My point was different - with this patch it is possible
> > that you are not adding any credits at all since the previous
> > VIRTIO_VSOCK_OP_CREDIT_UPDATE.
> 
> I think the problem we're solving here is that since as an optimization we
> avoid sending the update for every byte we consume, but we put a threshold,
> then we make sure we update the peer.
> 
> A credit update contains a snapshot and sending it the same as the previous
> one should not create any problem.

Well it consumes a buffer on the other side.

> My doubt now is that we only do this when we set RCVLOWAT , should we also
> do something when we consume bytes to avoid the optimization we have?
> 
> Stefano

Isn't this why we have credit request?

-- 
MST