netdev - [PATCH bpf] xdp: Fix spurious packet loss in generic XDP TX path

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Date:   Fri,  1 Jul 2022 11:42:56 +0200
From:   Johan Almbladh <johan.almbladh@...finetworks.com>
To:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com, hawk@...nel.org, john.fastabend@...il.com
Cc:     song@...nel.org, martin.lau@...ux.dev, yhs@...com,
        kpsingh@...nel.org, sdf@...gle.com, haoluo@...gle.com,
        jolsa@...nel.org, bpf@...r.kernel.org, netdev@...r.kernel.org,
        Johan Almbladh <johan.almbladh@...finetworks.com>
Subject: [PATCH bpf] xdp: Fix spurious packet loss in generic XDP TX path

The byte queue limits (BQL) mechanism is intended to move queuing from
the driver to the network stack in order to reduce latency caused by
excessive queuing in hardware. However, when transmitting or redirecting
a packet with XDP, the qdisc layer is bypassed and there are no
additional queues. Since netif_xmit_stopped() also takes BQL limits into
account, but without having any alternative queuing, packets are
silently dropped.

This patch modifies the drop condition to only consider cases when the
driver itself cannot accept any more packets. This is analogous to the
condition in __dev_direct_xmit(). Dropped packets are also counted on
the device.

Signed-off-by: Johan Almbladh <johan.almbladh@...finetworks.com>
---
 net/core/dev.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 8e6f22961206..41b5d7ac5ec5 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4875,10 +4875,12 @@ void generic_xdp_tx(struct sk_buff *skb, struct bpf_prog *xdp_prog)
 	txq = netdev_core_pick_tx(dev, skb, NULL);
 	cpu = smp_processor_id();
 	HARD_TX_LOCK(dev, txq, cpu);
-	if (!netif_xmit_stopped(txq)) {
+	if (!netif_xmit_frozen_or_drv_stopped(txq)) {
 		rc = netdev_start_xmit(skb, dev, txq, 0);
 		if (dev_xmit_complete(rc))
 			free_skb = false;
+	} else {
+		dev_core_stats_tx_dropped_inc(dev);
 	}
 	HARD_TX_UNLOCK(dev, txq);
 	if (free_skb) {
-- 
2.30.2