Re: [PATCH RFC net-next 1/2] tcp: Add DPIFL thin stream detection mechanism

On Fri, 2015-10-23 at 22:50 +0200, Bendik Rønning Opstad wrote:

 
+/**
+ * tcp_stream_is_thin_dpifl() - Tests if the stream is thin based on dynamic PIF
+ *                              limit
+ * @tp: the tcp_sock struct
+ *
+ * Return: true if current packets in flight (PIF) count is lower than
+ *         the dynamic PIF limit, else false
+ */
+static inline bool tcp_stream_is_thin_dpifl(const struct tcp_sock *tp)
+{
+	u64 dpif_lim = tp->srtt_us >> 3;
+	/* Div by is_thin_min_itt_lim, the minimum allowed ITT
+	 * (Inter-transmission time) in usecs.
+	 */
+	do_div(dpif_lim, tp->thin_dpifl_itt_lower_bound);
+	return tcp_packets_in_flight(tp) < dpif_lim;
+}
+

This is very strange :

You are using a do_div() while both operands are 32bits.  A regular
divide would be ok :

u32 dpif_lim = (tp->srtt_us >> 3) / tp->thin_dpifl_itt_lower_bound;

But then, you can avoid the divide by using a multiply, less expensive :

return	(u64)tcp_packets_in_flight(tp) * tp->thin_dpifl_itt_lower_bound <
	(tp->srtt_us >> 3);