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

From: "Bendik Rønning Opstad" <[email-masked]>
Date: Sun, 25 Oct 2015 06:56:29 +0100

On Friday, October 23, 2015 02:44:14 PM Eric Dumazet wrote:
> 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);
> 

You are of course correct. Will fix this and use multiply. Thanks.