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

[Paolo Abeni <pabeni@redhat.com>]

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`

[...]> +/* 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?

/P