From 1e6f74536de08b5e50cf0e37e735911c2cef7c62 Mon Sep 17 00:00:00 2001 From: Willem de Bruijn Date: Fri, 6 Oct 2017 13:22:31 -0400 Subject: vhost_net: do not stall on zerocopy depletion 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. Without rate limiting, {1, 10, 100}x TCP_STREAM tests continued to send at 100% zerocopy. The limit in vhost_exceeds_maxpend must be carefully chosen. With 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. Changes v1 -> v2 - replaced min with typed min_t - avoid unnecessary whitespace change 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 Signed-off-by: David S. Miller --- drivers/vhost/net.c | 12 +++--------- 1 file changed, 3 insertions(+), 9 deletions(-) (limited to 'drivers') diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c index 58585ec8699e..68677d930e20 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_t(unsigned int, VHOST_MAX_PEND, vq->num >> 2); } /* Expects to be always run from workqueue - which acts as @@ -480,11 +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), @@ -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 */ -- cgit v1.2.3