Re: [PATCH RFC mptcp-next 3/4] mptcp: round-robin packet scheduler support

[Paolo Abeni <pabeni@redhat.com>]

On Tue, 2021-09-07 at 18:41 +0800, Geliang Tang wrote:
> From: Geliang Tang <geliangtang@xiaomi.com>
> 
> Implement the round-robin packet scheduler like on the mptcp.org kernel.
> 
> Signed-off-by: Geliang Tang <geliangtang@xiaomi.com>
> ---
>  net/mptcp/Kconfig    |  7 +++++++
>  net/mptcp/protocol.c | 26 ++++++++++++++++++++++++++
>  2 files changed, 33 insertions(+)
> 
> diff --git a/net/mptcp/Kconfig b/net/mptcp/Kconfig
> index 10c97e19a7da..0df36991566a 100644
> --- a/net/mptcp/Kconfig
> +++ b/net/mptcp/Kconfig
> @@ -23,6 +23,13 @@ config MPTCP_IPV6
>  	depends on IPV6=y
>  	default y
>  
> +config MPTCP_ROUNDROBIN
> +	tristate "MPTCP Round-Robin"
> +	default n
> +	help
> +	  This is a very simple round-robin scheduler. Probably has bad performance
> +	  but might be interesting for researchers.
> +
>  config MPTCP_KUNIT_TEST
>  	tristate "This builds the MPTCP KUnit tests" if !KUNIT_ALL_TESTS
>  	depends on KUNIT
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index ab72a3950f2b..b78c4eb4947f 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -1521,6 +1521,29 @@ static struct mptcp_sched_ops mptcp_sched_default = {
>  	.owner		= THIS_MODULE,
>  };
>  
> +#if IS_ENABLED(CONFIG_MPTCP_ROUNDROBIN)
> +static struct sock *rr_get_subflow(struct mptcp_sock *msk)
> +{
> +	struct mptcp_subflow_context *subflow;
> +	struct sock *ssk;
> +
> +	mptcp_for_each_subflow(msk, subflow) {
> +		ssk = mptcp_subflow_tcp_sock(subflow);
> +		if (ssk != msk->last_snd) {
> +			pr_debug("msk=%p ssk=%p last_snd=%p", msk, ssk, msk->last_snd);
> +			msk->last_snd = ssk;
> +		}

This needs some more logic:
- it must take in account backup vs non backup
- it should consider if the given subflow has available write
space/snd_wnd
- it should move from the 'last_snd' subflow to the 'next' one, instead
of always selecting the fist subflow other then 'last_snd'. Otherwise
with 3 non backup subflow active we will never pick the last created
one.

There is an import side question, already asked by Christoph, why the
round robin? perhpas we should consider instead some different
scheduler, known to perform well. @Christoph: which is the 'best'
mptcp.org scheduler out there? blest?

Thanks!

Paolo