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

Bendik Rønning Opstad
2015-10-25T05:56:29Z

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.