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

[Simon Horman <horms@kernel.org>]

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---
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/

[ ... ]

> diff --git a/net/quic/cong.h b/net/quic/cong.h
> new file mode 100644
> index 000000000000..e6cfb0fa1b6c
> --- /dev/null
> +++ b/net/quic/cong.h

[ ... ]

> +struct quic_cong {
> +	/* RTT tracking */
> +	u32 max_ack_delay;	/* max_ack_delay from rfc9000#section-18.2 */
> +	u32 smoothed_rtt;	/* Smoothed RTT */
> +	u32 latest_rtt;		/* Latest RTT sample */
> +	u32 min_rtt;		/* Lowest observed RTT */
> +	u32 rttvar;		/* RTT variation */
> +	u32 pto;		/* Probe timeout */
> +
> +	/* Timing & pacing */
> +	u64 recovery_time;	/* Recovery period start timestamp */
> +	u64 pacing_rate;	/* Packet sending speed Bytes/sec */
> +	u64 pacing_time;	/* Next scheduled send timestamp (ns) */
> +	u64 time;		/* Cachedached current timestamp */
                                   ^^^^^^^^^^

There appears to be a typo here - "Cachedached" should be "Cached".