Re: [PATCH mptcp-next v3 3/4] mptcp: sched: do not penalise when receive-window-limited

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] The `mptcp_snd_wnd_test()` heuristic incorrectly disables the slow-subflow penalty for standard bulk transfers by equating application queue depth with receive-window limitation.
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--- Patch [3]: [PATCH mptcp-next v3 3/4] mptcp: sched: do not penalise when receive-window-limited ---
commit 97d5176c170e6dd87e10d2ec2eefd7e6a38340c4
Author: Shardul Bankar <shardul.b@mpiricsoftware.com>

mptcp: sched: do not penalise when receive-window-limited

This commit disables the slow-subflow cwnd penalty when the connection
is receive-window limited. It introduces mptcp_snd_wnd_test to skip the
penalty if the application has queued past the window edge.

> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -1635,6 +1635,14 @@ static bool mptcp_penalise_throttle_ok(struct mptcp_subflow_context *subflow)
>  	return tcp_jiffies32 - subflow->last_penalise >= max_t(u32, rtt, 1);
>  }
>  
> +/* Like tcp_snd_wnd_test() but without an skb: true while queued data still fits
> + * the send window, i.e. not receive-window-limited.
> + */
> +static bool mptcp_snd_wnd_test(const struct mptcp_sock *msk)
> +{
> +	return !after64(msk->write_seq, mptcp_wnd_end(msk));

[Severity: High]
During a standard bulk data transfer, the application typically pushes data
into the socket buffer up to sk_sndbuf, which is often much larger than the
peer's advertised receive window.

Because of this, won't msk->write_seq naturally exceed mptcp_wnd_end(msk),
causing this test to return false even when the network is bottlenecked by
a small congestion window rather than the receive window?

If this evaluates to false during bulk transfers, doesn't it unconditionally
disable the slow-subflow penalty and prevent the intended shift of load to
faster subflows?

> +}
> +
>  /* Halve cwnd (and ssthresh if past it) under the subflow socket lock. */
>  static void mptcp_penalise_cwnd(struct sock *ssk)

[ ... ]

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Sashiko AI review · https://sashiko.dev/#/patchset/20260817-mptcp_penalise_send_v2-v3-0-e6a2ad2f1b82@mpiricsoftware.com?part=3