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

[Jason Wang <jasowang@redhat.com>]

On 2017年09月28日 08:25, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.com>
>
> 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.

Have you tested e.g vq->num = 512 or 1024?

>   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=3ThTXa6BnZkygP3tgVpjbp93g@mail.gmail.com
> Link:http://lkml.kernel.org/r/20170819064129.27272-1-den@klaipeden.com
> Signed-off-by: Willem de Bruijn <willemb@google.com>
> ---
>   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);
> +

Looks unnecessary. Other looks good.

>   		/* 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 */