netdev - Re: [PATCH v6 net-next 15/19] ionic: Add Tx and Rx handling

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Message-ID: <20190829161852.1705d770@cakuba.netronome.com>
Date:   Thu, 29 Aug 2019 16:18:52 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Shannon Nelson <snelson@...sando.io>
Cc:     netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH v6 net-next 15/19] ionic: Add Tx and Rx handling

On Thu, 29 Aug 2019 11:27:16 -0700, Shannon Nelson wrote:
> +netdev_tx_t ionic_start_xmit(struct sk_buff *skb, struct net_device *netdev)
> +{
> +	u16 queue_index = skb_get_queue_mapping(skb);
> +	struct ionic_lif *lif = netdev_priv(netdev);
> +	struct ionic_queue *q;
> +	int ndescs;
> +	int err;
> +
> +	if (unlikely(!test_bit(IONIC_LIF_UP, lif->state))) {
> +		dev_kfree_skb(skb);
> +		return NETDEV_TX_OK;
> +	}
> +
> +	if (likely(lif_to_txqcq(lif, queue_index)))
> +		q = lif_to_txq(lif, queue_index);
> +	else
> +		q = lif_to_txq(lif, 0);
> +
> +	ndescs = ionic_tx_descs_needed(q, skb);
> +	if (ndescs < 0)
> +		goto err_out_drop;
> +
> +	if (!ionic_q_has_space(q, ndescs)) {

You should stop the queue in advance, whenever you can't ensure that a
max size frame can be placed on the ring. Requeuing is very expensive
so modern drivers should try to never return NETDEV_TX_BUSY

> +		netif_stop_subqueue(netdev, queue_index);
> +		q->stop++;
> +
> +		/* Might race with ionic_tx_clean, check again */
> +		smp_rmb();
> +		if (ionic_q_has_space(q, ndescs)) {
> +			netif_wake_subqueue(netdev, queue_index);
> +			q->wake++;
> +		} else {
> +			return NETDEV_TX_BUSY;
> +		}
> +	}
> +
> +	if (skb_is_gso(skb))
> +		err = ionic_tx_tso(q, skb);
> +	else
> +		err = ionic_tx(q, skb);
> +
> +	if (err)
> +		goto err_out_drop;
> +
> +	return NETDEV_TX_OK;
> +
> +err_out_drop:
> +	netif_stop_subqueue(netdev, queue_index);

This stopping of the queue is suspicious, if ionic_tx() fails there's
no guarantee the queue will ever be woken up, no?

> +	q->stop++;
> +	q->drop++;
> +	dev_kfree_skb(skb);
> +	return NETDEV_TX_OK;
> +}