Re: [PATCH net-next 1/3] veth: Account for packet drops in ndo_xdp_xmit

[Toshiaki Makita]

On 18/10/13 (土) 16:48, Jesper Dangaard Brouer wrote:
On Thu, 11 Oct 2018 18:36:48 +0900
Toshiaki Makita <makita.toshi...@lab.ntt.co.jp> wrote:
Use existing atomic drop counter. Since drop path is really an
exceptional case here, I'm thinking atomic ops would not hurt the
performance.

Hmm... we try very hard not to add atomic ops to XDP code path. The
XDP_DROP case is also considered hot-path.  In below code, the
atomic64_add happens for a bulk of dropped packets (currently up-to
16), so it might be okay.

Yes, this happens only once in a bulk sending.
Note that this drop does not include XDP_DROP. This drop is counted when
- ndo_xdp_xmit "flags" arg is invalid
- peer is detached
- XDP is not loaded on peer
- XDP ring (256 slots) overflow
So really exceptional. XDP_DROP is counted per-queue basis (non-atomic) 
in the patch 2/3.

Toshiaki Makita


XDP packets and bytes are not counted in ndo_xdp_xmit, but will be
accounted on rx side by the following commit.

Signed-off-by: Toshiaki Makita <makita.toshi...@lab.ntt.co.jp>
---
  drivers/net/veth.c | 30 ++++++++++++++++++++++--------
  1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 224c56a..452193f2 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -308,16 +308,20 @@ static int veth_xdp_xmit(struct net_device *dev, int n,
  {
        struct veth_priv *rcv_priv, *priv = netdev_priv(dev);
        struct net_device *rcv;
+       int i, ret, drops = n;
        unsigned int max_len;
        struct veth_rq *rq;
-       int i, drops = 0;
  
-	if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK))
-               return -EINVAL;
+       if (unlikely(flags & ~XDP_XMIT_FLAGS_MASK)) {
+               ret = -EINVAL;
+               goto drop;
+       }
  
  	rcv = rcu_dereference(priv->peer);
-       if (unlikely(!rcv))
-               return -ENXIO;
+       if (unlikely(!rcv)) {
+               ret = -ENXIO;
+               goto drop;
+       }
  
  	rcv_priv = netdev_priv(rcv);
        rq = &rcv_priv->rq[veth_select_rxq(rcv)];
@@ -325,9 +329,12 @@ static int veth_xdp_xmit(struct net_device *dev, int n,
         * side. This means an XDP program is loaded on the peer and the peer
         * device is up.
         */
-       if (!rcu_access_pointer(rq->xdp_prog))
-               return -ENXIO;
+       if (!rcu_access_pointer(rq->xdp_prog)) {
+               ret = -ENXIO;
+               goto drop;
+       }
  
+	drops = 0;
        max_len = rcv->mtu + rcv->hard_header_len + VLAN_HLEN;
  
  	spin_lock(&rq->xdp_ring.producer_lock);
@@ -346,7 +353,14 @@ static int veth_xdp_xmit(struct net_device *dev, int n,
        if (flags & XDP_XMIT_FLUSH)
                __veth_xdp_flush(rq);
  
-	return n - drops;
+       if (likely(!drops))
+               return n;
+
+       ret = n - drops;
+drop:
+       atomic64_add(drops, &priv->dropped);
+
+       return ret;
  }
  
  static void veth_xdp_flush(struct net_device *dev)