Subject: Re: [PATCH net-next] vhost_net: do not stall on zerocopy depletion

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 29 Sep 2017 22:38:04 +0300

On Wed, Sep 27, 2017 at 08:25:56PM -0400, Willem de Bruijn wrote:
> From: Willem de Bruijn <[email-masked]>
> 
> Vhost-net has a hard limit on the number of zerocopy skbs in flight.
> When reached, transmission stalls. Stalls cause latency, as well as
> head-of-line blocking of other flows that do not use zerocopy.
> 
> Instead of stalling, revert to copy-based transmission.
> 
> Tested by sending two udp flows from guest to host, one with payload
> of VHOST_GOODCOPY_LEN, the other too small for zerocopy (1B). The
> large flow is redirected to a netem instance with 1MBps rate limit
> and deep 1000 entry queue.
> 
>   modprobe ifb
>   ip link set dev ifb0 up
>   tc qdisc add dev ifb0 root netem limit 1000 rate 1MBit
> 
>   tc qdisc add dev tap0 ingress
>   tc filter add dev tap0 parent ffff: protocol ip \
>       u32 match ip dport 8000 0xffff \
>       action mirred egress redirect dev ifb0
> 
> Before the delay, both flows process around 80K pps. With the delay,
> before this patch, both process around 400. After this patch, the
> large flow is still rate limited, while the small reverts to its
> original rate. See also discussion in the first link, below.
> 
> The limit in vhost_exceeds_maxpend must be carefully chosen. When
> vq->num >> 1, the flows remain correlated. This value happens to
> correspond to VHOST_MAX_PENDING for vq->num == 256. Allow smaller
> fractions and ensure correctness also for much smaller values of
> vq->num, by testing the min() of both explicitly. See also the
> discussion in the second link below.
> 
> Link:http://lkml.kernel.org/r/CAF=yD-+Wk9sc9dXMUq1+x_hh=[email-masked]
> Link:http://lkml.kernel.org/r/[email-masked]
> Signed-off-by: Willem de Bruijn <[email-masked]>

I'd like to see the effect on the non rate limited case though.
If guest is quick won't we have lots of copies then?

> ---
>  drivers/vhost/net.c | 14 ++++----------
>  1 file changed, 4 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 58585ec8699e..50758602ae9d 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -436,8 +436,8 @@ static bool vhost_exceeds_maxpend(struct vhost_net *net)
>  	struct vhost_net_virtqueue *nvq = &net->vqs[VHOST_NET_VQ_TX];
>  	struct vhost_virtqueue *vq = &nvq->vq;
>  
> -	return (nvq->upend_idx + vq->num - VHOST_MAX_PEND) % UIO_MAXIOV
> -		== nvq->done_idx;
> +	return (nvq->upend_idx + UIO_MAXIOV - nvq->done_idx) % UIO_MAXIOV >
> +	       min(VHOST_MAX_PEND, vq->num >> 2);
>  }
>  
>  /* Expects to be always run from workqueue - which acts as
> @@ -480,12 +480,6 @@ static void handle_tx(struct vhost_net *net)
>  		if (zcopy)
>  			vhost_zerocopy_signal_used(net, vq);
>  
> -		/* If more outstanding DMAs, queue the work.
> -		 * Handle upend_idx wrap around
> -		 */
> -		if (unlikely(vhost_exceeds_maxpend(net)))
> -			break;
> -
>  		head = vhost_net_tx_get_vq_desc(net, vq, vq->iov,
>  						ARRAY_SIZE(vq->iov),
>  						&out, &in);
> @@ -509,6 +503,7 @@ static void handle_tx(struct vhost_net *net)
>  		len = iov_length(vq->iov, out);
>  		iov_iter_init(&msg.msg_iter, WRITE, vq->iov, out, len);
>  		iov_iter_advance(&msg.msg_iter, hdr_size);
> +
>  		/* Sanity check */
>  		if (!msg_data_left(&msg)) {
>  			vq_err(vq, "Unexpected header len for TX: "
> @@ -519,8 +514,7 @@ static void handle_tx(struct vhost_net *net)
>  		len = msg_data_left(&msg);
>  
>  		zcopy_used = zcopy && len >= VHOST_GOODCOPY_LEN
> -				   && (nvq->upend_idx + 1) % UIO_MAXIOV !=
> -				      nvq->done_idx
> +				   && !vhost_exceeds_maxpend(net)
>  				   && vhost_net_tx_select_zcopy(net);
>  
>  		/* use msg_control to pass vhost zerocopy ubuf info to skb */
> -- 
> 2.14.2.822.g60be5d43e6-goog