Re: [PATCH bpf-next 3/3] bpf: tcp: Add bpf_cubic example

On 1/21/20 11:54 AM, Martin KaFai Lau wrote:

This patch adds a bpf_cubic example.  Some highlights:
1. CONFIG_HZ kconfig is used.  For example, CONFIG_HZ is used in the usecs
   to jiffies conversion in usecs_to_jiffies().
2. In bitctcp_update() [under tcp_friendliness], the original
   "while (ca->ack_cnt > delta)" loop is changed to the equivalent
   "ca->ack_cnt / delta" operation

...

+	/* cubic function - calc*/
+	/* calculate c * time^3 / rtt,
+	 *  while considering overflow in calculation of time^3
+	 * (so time^3 is done by using 64 bit)
+	 * and without the support of division of 64bit numbers
+	 * (so all divisions are done by using 32 bit)
+	 *  also NOTE the unit of those veriables
+	 *	  time  = (t - K) / 2^bictcp_HZ
+	 *	  c = bic_scale >> 10
+	 * rtt  = (srtt >> 3) / HZ
+	 * !!! The following code does not have overflow problems,
+	 * if the cwnd < 1 million packets !!!
+	 */
+
+	t = (__s32)(tcp_jiffies32 - ca->epoch_start);
+	t += usecs_to_jiffies(ca->delay_min);
+	/* change the unit from HZ to bictcp_HZ */
+	t <<= BICTCP_HZ;
+	t /= HZ;

Note that this part could use usec resolution instead of jiffies
to avoid all these inlines for {u|n}secs_to_jiffies() 

	t = (__s32)(tcp_jiffies32 - ca->epoch_start) * (USEC_PER_JIFFY);
	t += ca->delay_min;
	/* change the unit from usec to bictcp_HZ */
	t <<= BICTCP_HZ;
	t /= USEC_PER_SEC;

ie :

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 8f8eefd3a3ce116aa8fa2b7ef85c7eb503fa8da7..9ba58e95dbe6b15098bcfd045e1d0bb8874d713f 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c

@@ -271,11 +271,11 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
         * if the cwnd < 1 million packets !!!
         */
 
-       t = (s32)(tcp_jiffies32 - ca->epoch_start);
-       t += usecs_to_jiffies(ca->delay_min);
-       /* change the unit from HZ to bictcp_HZ */
+       t = (s32)(tcp_jiffies32 - ca->epoch_start) * (USEC_PER_SEC / HZ);
+       t += ca->delay_min;
+       /* change the unit from usec to bictcp_HZ */
        t <<= BICTCP_HZ;
-       do_div(t, HZ);
+       do_div(t, USEC_PER_SEC);
 
        if (t < ca->bic_K)              /* t - K */
                offs = ca->bic_K - t;

But this is a minor detail.