Re: [PATCH net-next v4 09/15] quic: add congestion control

On Tue, Nov 4, 2025 at 7:02 AM Paolo Abeni <[email protected]> wrote:
>
> On 10/29/25 3:35 PM, Xin Long wrote:
> > +/* Compute and update the pacing rate based on congestion window and smoothed RTT. */
> > +static void quic_cong_pace_update(struct quic_cong *cong, u32 bytes, u32 max_rate)
> > +{
> > +     u64 rate;
> > +
> > +     /* rate = N * congestion_window / smoothed_rtt */
> > +     rate = (u64)cong->window * USEC_PER_SEC * 2;
> > +     if (likely(cong->smoothed_rtt))
> > +             rate = div64_ul(rate, cong->smoothed_rtt);
> > +
> > +     WRITE_ONCE(cong->pacing_rate, min_t(u64, rate, max_rate));
> > +     pr_debug("%s: update pacing rate: %u, max rate: %u, srtt: %u\n",
> > +              __func__, cong->pacing_rate, max_rate, cong->smoothed_rtt);
>
> I think you should skip entirely the pacing_rate update when
> `smoothed_rtt == 0`
>
will update it.

> [...]> +/* rfc9002#section-5: Estimating the Round-Trip Time */
> > +void quic_cong_rtt_update(struct quic_cong *cong, u32 time, u32 ack_delay)
> > +{
> > +     u32 adjusted_rtt, rttvar_sample;
> > +
> > +     /* Ignore RTT sample if ACK delay is suspiciously large. */
> > +     if (ack_delay > cong->max_ack_delay * 2)
> > +             return;
> > +
> > +     /* rfc9002#section-5.1: latest_rtt = ack_time - send_time_of_largest_acked */
> > +     cong->latest_rtt = cong->time - time;
> > +
> > +     /* rfc9002#section-5.2: Estimating min_rtt */
> > +     if (!cong->min_rtt_valid) {
> > +             cong->min_rtt = cong->latest_rtt;
> > +             cong->min_rtt_valid = 1;
> > +     }
> > +     if (cong->min_rtt > cong->latest_rtt)
> > +             cong->min_rtt = cong->latest_rtt;
> > +
> > +     if (!cong->is_rtt_set) {
> > +             /* rfc9002#section-5.3:
> > +              *   smoothed_rtt = latest_rtt
> > +              *   rttvar = latest_rtt / 2
> > +              */
> > +             cong->smoothed_rtt = cong->latest_rtt;
> > +             cong->rttvar = cong->smoothed_rtt / 2;
> > +             quic_cong_pto_update(cong);
> > +             cong->is_rtt_set = 1;
> > +             return;
> > +     }
> > +
> > +     /* 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;
>
> Out of sheer curiosity, is the compiler smart enough to use a 'srl 3'
> for the above?
>
Yes.

266 cong->smoothed_rtt = (cong->smoothed_rtt * 7 + adjusted_rtt) / 8;

0x593d <+77>:  mov    (%rbx),%ecx         ; ecx = cong->smoothed_rtt
0x593f <+79>:  lea    (%rax,%rcx,8),%edx   ; edx = adjusted_rtt + (ecx * 8)
0x5942 <+82>:  sub    %ecx,%edx           ; edx = adjusted_rtt +
(8*ecx) - ecx = ecx*7 + adjusted_rtt
0x5946 <+86>:  shr    $0x3,%edx           ; edx >>= 3 → divide by 8
0x594d <+93>:  mov    %edx,(%rbx)         ; store result back to
cong->smoothed_rtt

Thanks.