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