Re: [net-next,v8,09/15] quic: add congestion control

[Simon Horman <horms@kernel.org>]

On Thu, Jan 29, 2026 at 02:44:24PM -0500, Xin Long wrote:
> On Wed, Jan 28, 2026 at 11:15 AM Simon Horman <horms@kernel.org> wrote:
> >
> > This is an AI-generated review of your patch. The human sending this
> > email has considered the AI review valid, or at least plausible.
> >
> > For local reproduction steps see https://netdev-ai.bots.linux.dev/ai-local.html
> > ---
> > quic: add congestion control
> >
> > This patch introduces quic_cong for RTT measurement and congestion
> > control. It implements a congestion control state machine with slow
> > start, congestion avoidance, and recovery phases using New Reno.
> >
> > > diff --git a/net/quic/cong.c b/net/quic/cong.c
> > > new file mode 100644
> > > index 000000000000..1a8b7f8db977
> > > --- /dev/null
> > > +++ b/net/quic/cong.c
> >
> > [ ... ]
> >
> > > +/* rfc9002#section-5: Estimating the Round-Trip Time */
> > > +void quic_cong_rtt_update(struct quic_cong *cong, u64 time, u32 ack_delay)
> > > +{
> > > +     u32 adjusted_rtt, rttvar_sample;
> >
> > [ ... ]
> >
> > > +     /* rfc9002#section-5.3:
> > > +      *   adjusted_rtt = latest_rtt
> > > +      *   if (latest_rtt >= min_rtt + ack_delay):
> > > +      *     adjusted_rtt = latest_rtt - ack_delay
> > > +      *   smoothed_rtt = 7/8 * smoothed_rtt + 1/8 * adjusted_rtt
> > > +      *   rttvar_sample = abs(smoothed_rtt - adjusted_rtt)
> > > +      *   rttvar = 3/4 * rttvar + 1/4 * rttvar_sample
> > > +      */
> > > +     adjusted_rtt = cong->latest_rtt;
> > > +     if (cong->latest_rtt >= cong->min_rtt + ack_delay)
> > > +             adjusted_rtt = cong->latest_rtt - ack_delay;
> > > +
> > > +     cong->smoothed_rtt = (cong->smoothed_rtt * 7 + adjusted_rtt) / 8;
> > > +     if (cong->smoothed_rtt >= adjusted_rtt)
> > > +             rttvar_sample = cong->smoothed_rtt - adjusted_rtt;
> > > +     else
> > > +             rttvar_sample = adjusted_rtt - cong->smoothed_rtt;
> >
> > The comment above references abs(smoothed_rtt - adjusted_rtt). Paolo Abeni
> > suggested using abs_diff() from linux/math.h for this calculation in v3
> > review, and it was acknowledged. Could abs_diff() be used here instead of
> > the manual if-else?
> >
> > https://lore.kernel.org/netdev/a9427359-a798-4f3a-88ef-c10a0bf614ec@redhat.com/
> >
> Wow, AI truly has a good memory.

Yes, I was surprised too.

> Right, I somehow missed this. will apply abs_diff() here.

Thanks!

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