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SubjectRe: [PATCH] sctp: set ooo_okay properly for Transmit Packet Steering
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On 10/30/19 9:07 AM, Wally Zhao wrote:
> Unlike tcp_transmit_skb,
> sctp_packet_transmit does not set ooo_okay explicitly,
> causing unwanted Tx queue switching when multiqueue is in use;
> Tx queue switching may cause out-of-order packets.
> Change sctp_packet_transmit to allow Tx queue switching only for
> the first in flight packet, to avoid unwanted Tx queue switching.
>

While the patch seems fine, the changelog is quite confusing.

When skb->ooo_olay is 0 (which is the default for freshly allocated skbs),
the core networking stack will stick to whatever TX queue was chosen
at the time the dst_entry was attached to the (connected) socket.

This means no reorder can happen at all by default.

By setting ooo_okay carefully (as you did in your patch), you allow
core networking stack to _switch_ to another TX queue based on
current CPU (XPS selection)

So even without your fix, SCTP should not experience out-of-order packets.

> Signed-off-by: Wally Zhao <wallyzhao@gmail.com>
> ---
> net/sctp/output.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/net/sctp/output.c b/net/sctp/output.c
> index dbda7e7..5ff75cc 100644
> --- a/net/sctp/output.c
> +++ b/net/sctp/output.c
> @@ -626,6 +626,10 @@ int sctp_packet_transmit(struct sctp_packet *packet, gfp_t gfp)
> /* neighbour should be confirmed on successful transmission or
> * positive error
> */
> +
> + /* allow switch tx queue only for the first in flight pkt */
> + head->ooo_okay = asoc->outqueue.outstanding_bytes == 0;
> +
> if (tp->af_specific->sctp_xmit(head, tp) >= 0 &&
> tp->dst_pending_confirm)
> tp->dst_pending_confirm = 0;
>

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