netdev - Re: [PATCH net-next] tcp: ensure to use the most recently sent skb when filling the rate sample

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Date:   Thu, 14 Apr 2022 12:11:36 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Pengcheng Yang <yangpc@...gsu.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: ensure to use the most recently sent skb
 when filling the rate sample

On Wed, Apr 13, 2022 at 6:54 AM Pengcheng Yang <yangpc@...gsu.com> wrote:
>
> If an ACK (s)acks multiple skbs, we favor the information
> from the most recently sent skb by choosing the skb with
> the highest prior_delivered count.
> But prior_delivered may be equal, because tp->delivered only
> changes when receiving, which requires further comparison of
> skb timestamp.
>
> Signed-off-by: Pengcheng Yang <yangpc@...gsu.com>
> ---
>  net/ipv4/tcp_rate.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_rate.c b/net/ipv4/tcp_rate.c
> index 617b818..ad893ad 100644
> --- a/net/ipv4/tcp_rate.c
> +++ b/net/ipv4/tcp_rate.c
> @@ -86,7 +86,9 @@ void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb,
>                 return;
>
>         if (!rs->prior_delivered ||
> -           after(scb->tx.delivered, rs->prior_delivered)) {
> +           after(scb->tx.delivered, rs->prior_delivered) ||
> +           (scb->tx.delivered == rs->prior_delivered &&
> +            tcp_skb_timestamp_us(skb) > tp->first_tx_mstamp)) {
>                 rs->prior_delivered_ce  = scb->tx.delivered_ce;
>                 rs->prior_delivered  = scb->tx.delivered;
>                 rs->prior_mstamp     = scb->tx.delivered_mstamp;
> --

Thank you for posting this patch! I agree there is a bug there, and
your patch is an improvement. However, I think this patch is not a
complete solution, since it does not handle the case where there are
multiple skbs with the tcp_skb_timestamp_us() (which can happen if a
outgoing buffered TSO/GSO skb is later split into multiple skbs with
the same timestamp).

RACK has to deal with the same question "which skb was sent first?",
and already has a solution in tcp_rack_sent_after(). I suggest we
share code between RACK and tcp_rate_skb_delivered() to make this
check. This might involve making a copy of tcp_rack_sent_after() in
include/net/tcp.h, naming the .h copy to tcp_skb_sent_after(), and
reworking this logic to save and use the sequence number and timestamp
so that it can use the new tcp_skb_sent_after() helper. After this fix
propagates to net-next we could later then change RACK to use the new
tcp_skb_sent_after() function, so we have a single helper used in two
places.

If you want to post a version of this patch that uses some approach
like that, IMHO that would be welcome. If you do not have cycles, I am
happy to post one when I get a moment.

thanks,
neal