netdev - Re: [PATCH bpf-next V2] bpf: add skb->queue_mapping write access from tc clsact

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Date:   Tue, 19 Feb 2019 21:59:17 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH bpf-next V2] bpf: add skb->queue_mapping write access from
 tc clsact

On 02/19/2019 07:53 PM, Jesper Dangaard Brouer wrote:
> The skb->queue_mapping already have read access, via __sk_buff->queue_mapping.
> 
> This patch allow BPF tc qdisc clsact write access to the queue_mapping via
> tc_cls_act_is_valid_access.  Also handle that the value NO_QUEUE_MAPPING
> is not allowed.
> 
> It is already possible to change this via TC filter action skbedit
> tc-skbedit(8).  Due to the lack of TC examples, lets show one:
> 
>   # tc qdisc  add  dev ixgbe1 clsact
>   # tc filter add  dev ixgbe1 ingress matchall action skbedit queue_mapping 5
>   # tc filter list dev ixgbe1 ingress
> 
> The most common mistake is that XPS (Transmit Packet Steering) takes
> precedence over setting skb->queue_mapping. XPS is configured per DEVICE
> via /sys/class/net/DEVICE/queues/tx-*/xps_cpus via a CPU hex mask. To
> disable set mask=00.
> 
> The purpose of changing skb->queue_mapping is to influence the selection of
> the net_device "txq" (struct netdev_queue), which influence selection of
> the qdisc "root_lock" (via txq->qdisc->q.lock) and txq->_xmit_lock. When
> using the MQ qdisc the txq->qdisc points to different qdiscs and associated
> locks, and HARD_TX_LOCK (txq->_xmit_lock), allowing for CPU scalability.
> 
> Due to lack of TC examples, lets show howto attach clsact BPF programs:
> 
>  # tc qdisc  add  dev ixgbe2 clsact
>  # tc filter add  dev ixgbe2 egress bpf da obj XXX_kern.o sec tc_qmap2cpu
>  # tc filter list dev ixgbe2 egress
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>

Applied, thanks!