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Date: Thu, 24 Apr 2025 07:23:52 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Jesper Dangaard Brouer <hawk@kernel.org>
Cc: netdev@vger.kernel.org, bpf@vger.kernel.org, tom@herbertland.com, Eric
 Dumazet <eric.dumazet@gmail.com>, "David S. Miller" <davem@davemloft.net>,
 Paolo Abeni <pabeni@redhat.com>, Toke =?UTF-8?B?SMO4aWxhbmQtSsO4cmdlbnNl?=
 =?UTF-8?B?bg==?= <toke@toke.dk>, dsahern@kernel.org,
 makita.toshiaki@lab.ntt.co.jp, kernel-team@cloudflare.com, phil@nwl.cc
Subject: Re: [PATCH net-next V6 2/2] veth: apply qdisc backpressure on full
 ptr_ring to reduce TX drops
Message-ID: <20250424072352.18aa0df1@kernel.org>
In-Reply-To: <174549940981.608169.4363875844729313831.stgit@firesoul>
References: <174549933665.608169.392044991754158047.stgit@firesoul>
	<174549940981.608169.4363875844729313831.stgit@firesoul>
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On Thu, 24 Apr 2025 14:56:49 +0200 Jesper Dangaard Brouer wrote:
> +	case NETDEV_TX_BUSY:
> +		/* If a qdisc is attached to our virtual device, returning
> +		 * NETDEV_TX_BUSY is allowed.
> +		 */
> +		txq = netdev_get_tx_queue(dev, rxq);
> +
> +		if (qdisc_txq_has_no_queue(txq)) {
> +			dev_kfree_skb_any(skb);
> +			goto drop;
> +		}
> +		netif_tx_stop_queue(txq);
> +		/* Restore Eth hdr pulled by dev_forward_skb/eth_type_trans */
> +		__skb_push(skb, ETH_HLEN);
> +		/* Depend on prior success packets started NAPI consumer via
> +		 * __veth_xdp_flush(). Cancel TXQ stop if consumer stopped,
> +		 * paired with empty check in veth_poll().
> +		 */
> +		if (unlikely(__ptr_ring_empty(&rq->xdp_ring)))
> +			netif_tx_wake_queue(txq);

Looks like I wrote a reply to v5 but didn't hit send. But I may have
set v5 to Changes Requested because of it :S Here is my comment:

 I think this is missing a memory barrier. When drivers do this dance
 there's usually a barrier between stop and recheck, to make sure the
 stop is visible before we check. And vice versa veth_xdp_rcv() needs
 to make sure other side sees the "empty" indication before it checks 
 if the queue is stopped.