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.
