netdev - Re: [net-next v10 1/2] net: sched: use queue

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Date:   Mon, 14 Mar 2022 22:59:46 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     xiangxia.m.yue@...il.com, netdev@...r.kernel.org
Cc:     Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Alexander Lobakin <alobakin@...me>,
        Paolo Abeni <pabeni@...hat.com>,
        Talal Ahmad <talalahmad@...gle.com>,
        Kevin Hao <haokexin@...il.com>, ast@...nel.org,
        bpf@...r.kernel.org
Subject: Re: [net-next v10 1/2] net: sched: use queue_mapping to pick tx queue

On 3/14/22 3:15 PM, xiangxia.m.yue@...il.com wrote:
[...]
>   include/linux/netdevice.h |  3 +++
>   include/linux/rtnetlink.h |  1 +
>   net/core/dev.c            | 31 +++++++++++++++++++++++++++++--
>   net/sched/act_skbedit.c   |  6 +++++-
>   4 files changed, 38 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 0d994710b335..f33fb2d6712a 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3065,6 +3065,9 @@ struct softnet_data {
>   	struct {
>   		u16 recursion;
>   		u8  more;
> +#ifdef CONFIG_NET_EGRESS
> +		u8  skip_txqueue;
> +#endif
>   	} xmit;
>   #ifdef CONFIG_RPS
>   	/* input_queue_head should be written by cpu owning this struct,
> diff --git a/include/linux/rtnetlink.h b/include/linux/rtnetlink.h
> index 7f970b16da3a..ae2c6a3cec5d 100644
> --- a/include/linux/rtnetlink.h
> +++ b/include/linux/rtnetlink.h
> @@ -100,6 +100,7 @@ void net_dec_ingress_queue(void);
>   #ifdef CONFIG_NET_EGRESS
>   void net_inc_egress_queue(void);
>   void net_dec_egress_queue(void);
> +void netdev_xmit_skip_txqueue(bool skip);
>   #endif
>   
>   void rtnetlink_init(void);
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 75bab5b0dbae..8e83b7099977 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3908,6 +3908,25 @@ sch_handle_egress(struct sk_buff *skb, int *ret, struct net_device *dev)
>   
>   	return skb;
>   }
> +
> +static struct netdev_queue *
> +netdev_tx_queue_mapping(struct net_device *dev, struct sk_buff *skb)
> +{
> +	int qm = skb_get_queue_mapping(skb);
> +
> +	return netdev_get_tx_queue(dev, netdev_cap_txqueue(dev, qm));
> +}
> +
> +static bool netdev_xmit_txqueue_skipped(void)
> +{
> +	return __this_cpu_read(softnet_data.xmit.skip_txqueue);
> +}
> +
> +void netdev_xmit_skip_txqueue(bool skip)
> +{
> +	__this_cpu_write(softnet_data.xmit.skip_txqueue, skip);
> +}
> +EXPORT_SYMBOL_GPL(netdev_xmit_skip_txqueue);
>   #endif /* CONFIG_NET_EGRESS */
>   
>   #ifdef CONFIG_XPS
> @@ -4078,7 +4097,7 @@ struct netdev_queue *netdev_core_pick_tx(struct net_device *dev,
>   static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
>   {
>   	struct net_device *dev = skb->dev;
> -	struct netdev_queue *txq;
> +	struct netdev_queue *txq = NULL;
>   	struct Qdisc *q;
>   	int rc = -ENOMEM;
>   	bool again = false;
> @@ -4106,11 +4125,17 @@ static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
>   			if (!skb)
>   				goto out;
>   		}
> +
> +		netdev_xmit_skip_txqueue(false);
> +
>   		nf_skip_egress(skb, true);
>   		skb = sch_handle_egress(skb, &rc, dev);
>   		if (!skb)
>   			goto out;
>   		nf_skip_egress(skb, false);
> +
> +		if (netdev_xmit_txqueue_skipped())
> +			txq = netdev_tx_queue_mapping(dev, skb);
>   	}
>   #endif
>   	/* If device/qdisc don't need skb->dst, release it right now while
> @@ -4121,7 +4146,9 @@ static int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
>   	else
>   		skb_dst_force(skb);
>   
> -	txq = netdev_core_pick_tx(dev, skb, sb_dev);
> +	if (likely(!txq))

nit: Drop likely(). If the feature is used from sch_handle_egress(), then this would always be the case.

> +		txq = netdev_core_pick_tx(dev, skb, sb_dev);
> +
>   	q = rcu_dereference_bh(txq->qdisc);

How will the `netdev_xmit_skip_txqueue(true)` be usable from BPF side (see bpf_convert_ctx_access() ->
queue_mapping)?

>   	trace_net_dev_queue(skb);
> diff --git a/net/sched/act_skbedit.c b/net/sched/act_skbedit.c
> index ceba11b198bb..d5799b4fc499 100644
> --- a/net/sched/act_skbedit.c
> +++ b/net/sched/act_skbedit.c
> @@ -58,8 +58,12 @@ static int tcf_skbedit_act(struct sk_buff *skb, const struct tc_action *a,
>   		}
>   	}
>   	if (params->flags & SKBEDIT_F_QUEUE_MAPPING &&
> -	    skb->dev->real_num_tx_queues > params->queue_mapping)
> +	    skb->dev->real_num_tx_queues > params->queue_mapping) {
> +#ifdef CONFIG_NET_EGRESS
> +		netdev_xmit_skip_txqueue(true);
> +#endif
>   		skb_set_queue_mapping(skb, params->queue_mapping);
> +	}
>   	if (params->flags & SKBEDIT_F_MARK) {
>   		skb->mark &= ~params->mask;
>   		skb->mark |= params->mark & params->mask;
>