netdev - [PATCH bpf] xsk: do not discard packet when QUEUE_STATE

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Message-Id: <1594287951-27479-1-git-send-email-magnus.karlsson@intel.com>
Date:   Thu,  9 Jul 2020 11:45:51 +0200
From:   Magnus Karlsson <magnus.karlsson@...el.com>
To:     magnus.karlsson@...el.com, bjorn.topel@...el.com, ast@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org,
        jonathan.lemon@...il.com
Cc:     A.Zema@...convsystems.com
Subject: [PATCH bpf] xsk: do not discard packet when QUEUE_STATE_FROZEN

In the skb Tx path, transmission of a packet is performed with
dev_direct_xmit(). When QUEUE_STATE_FROZEN is set in the transmit
routines, it returns NETDEV_TX_BUSY signifying that it was not
possible to send the packet now, please try later. Unfortunately, the
xsk transmit code discarded the packet and returned EBUSY to the
application. Fix this unnecessary packet loss, by not discarding the
packet and return EAGAIN. As EAGAIN is returned to the application, it
can then retry the send operation and the packet will finally be sent
as we will likely not be in the QUEUE_STATE_FROZEN state anymore. So
EAGAIN tells the application that the packet was not discarded from
the Tx ring and that it needs to call send() again. EBUSY, on the
other hand, signifies that the packet was not sent and discarded from
the Tx ring. The application needs to put the packet on the Tx ring
again if it wants it to be sent.

This unnecessary packet loss has been reported by the user below to
occur at high transmit loads.

Fixes: 35fcde7f8deb ("xsk: support for Tx")
Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
Reported-by: Arkadiusz Zema <A.Zema@...convsystems.com>
Suggested-by: Arkadiusz Zema <A.Zema@...convsystems.com>
---
 net/xdp/xsk.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 3700266..5304250 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -376,13 +376,22 @@ static int xsk_generic_xmit(struct sock *sk)
 		skb->destructor = xsk_destruct_skb;

 		err = dev_direct_xmit(skb, xs->queue_id);
-		xskq_cons_release(xs->tx);
 		/* Ignore NET_XMIT_CN as packet might have been sent */
-		if (err == NET_XMIT_DROP || err == NETDEV_TX_BUSY) {
+		if (err == NET_XMIT_DROP) {
 			/* SKB completed but not sent */
+			xskq_cons_release(xs->tx);
 			err = -EBUSY;
 			goto out;
+		} else if  (err == NETDEV_TX_BUSY) {
+			/* QUEUE_STATE_FROZEN, tell application to
+			 * retry sending the packet
+			 */
+			skb->destructor = NULL;
+			kfree_skb(skb);
+			err = -EAGAIN;
+			goto out;
 		}
+		xskq_cons_release(xs->tx);

 		sent_frame = true;
 	}
-- 
2.7.4