[MPTCP] Re: [PATCH mptcp-next 2/2] mptcp: add receive buffer auto-tuning

[Florian Westphal <fw at strlen.de>]

[-- Attachment #1: Type: text/plain, Size: 1343 bytes --]

Paolo Abeni <pabeni(a)redhat.com> wrote:
> On Thu, 2020-05-28 at 15:02 +0200, Florian Westphal wrote:
> > +static void mptcp_rcv_space_adjust(struct mptcp_sock *msk, int copied)
> > +{
> > +	struct mptcp_subflow_context *subflow;
> > +	struct sock *sk = (struct sock *)msk;
> > +	u32 time, advmss = 1;
> > +	u64 rtt_us, mstamp;
> > +
> > +	sock_owned_by_me(sk);
> > +
> > +	if (copied <= 0)
> > +		return;
> > +
> > +	msk->rcvq_space.copied += copied;
> > +
> > +	rtt_us = 0;
> > +	mptcp_for_each_subflow(msk, subflow) {
> > +		const struct tcp_sock *tp;
> > +		u64 sf_rtt_us;
> > +		u32 sf_advmss;
> > +
> > +		tp = tcp_sk(mptcp_subflow_tcp_sock(subflow));
> > +
> > +		sf_rtt_us = READ_ONCE(tp->rcv_rtt_est.rtt_us);
> > +		sf_advmss = READ_ONCE(tp->advmss);
> > +
> > +		rtt_us = max(sf_rtt_us, rtt_us);
> > +		advmss = max(sf_advmss, advmss);
> > +	}
> > +
> > +	mstamp = div_u64(tcp_clock_ns(), NSEC_PER_USEC);
> > +
> > +	time = tcp_stamp_us_delta(mstamp, msk->rcvq_space.time);
> > +	if (time < (rtt_us >> 3) || rtt_us == 0)
> > +		return;
> 
> what if you cache the rtt_ns value, too, and use it to move the above
> test before the loop? So that the receive path will loop on the subflow
> at most once every rtt_us/8.

Which rtt_us should I cache?  The one used during last measurement
window?