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

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Message-ID: <11d2572e-3ff5-2fc0-8f05-c50dd0fb1d6d@iogearbox.net>
Date:   Tue, 19 Feb 2019 12:46:57 +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] bpf: add skb->queue_mapping write access from tc
 clsact

On 02/19/2019 11:24 AM, 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.
> 
> 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 handle ffff: ingress
>  # tc filter add  dev ixgbe1 parent ffff: matchall action skbedit queue_mapping 5
>  # tc filter list dev ixgbe1 parent ffff:

Using handles was in the old days, if we add examples, then lets do
something more user friendly ;)

  # tc qdisc  add     dev ixgbe1 clsact
  # tc filter replace 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 replace 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>
> ---
>  net/core/filter.c |   14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 353735575204..d05ae8d05397 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -6238,6 +6238,7 @@ static bool tc_cls_act_is_valid_access(int off, int size,
>  		case bpf_ctx_range(struct __sk_buff, tc_classid):
>  		case bpf_ctx_range_till(struct __sk_buff, cb[0], cb[4]):
>  		case bpf_ctx_range(struct __sk_buff, tstamp):
> +		case bpf_ctx_range(struct __sk_buff, queue_mapping):
>  			break;
>  		default:
>  			return false;
> @@ -6642,9 +6643,16 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
>  		break;
>  
>  	case offsetof(struct __sk_buff, queue_mapping):
> -		*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
> -				      bpf_target_off(struct sk_buff, queue_mapping, 2,
> -						     target_size));
> +		if (type == BPF_WRITE)
> +			*insn++ = BPF_STX_MEM(BPF_H, si->dst_reg, si->src_reg,
> +					      bpf_target_off(struct sk_buff,
> +							     queue_mapping,
> +							     2, target_size));
> +		else
> +			*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
> +					      bpf_target_off(struct sk_buff,
> +							     queue_mapping,
> +							     2, target_size));

One thing we should avoid would be to allow user to write NO_QUEUE_MAPPING
into skb->queue_mapping so we don't hit the warn in sk_tx_queue_set(), I'd
add this into the ctx rewrite here.

>  		break;
>  
>  	case offsetof(struct __sk_buff, vlan_present):
>