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

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Date:	Fri, 23 Oct 2015 14:44:14 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Bendik Rønning Opstad <bro.devel@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	Jonathan Corbet <corbet@....net>,
	Eric Dumazet <edumazet@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Tom Herbert <tom@...bertland.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>, Erik Kline <ek@...gle.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jiri Pirko <jiri@...nulli.us>,
	Alexander Duyck <alexander.h.duyck@...hat.com>,
	Florian Westphal <fw@...len.de>,
	Daniel Lee <Longinus00@...il.com>,
	Marcelo Ricardo Leitner <mleitner@...hat.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Willem de Bruijn <willemb@...gle.com>,
	Linus Lüssing <linus.luessing@...3.blue>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, linux-api@...r.kernel.org,
	Andreas Petlund <apetlund@...ula.no>,
	Carsten Griwodz <griff@...ula.no>,
	Pål Halvorsen <paalh@...ula.no>,
	Jonas Markussen <jonassm@....uio.no>,
	Kristian Evensen <kristian.evensen@...il.com>,
	Kenneth Klette Jonassen <kennetkl@....uio.no>,
	Bendik Rønning Opstad 
	<bro.devel+kernel@...il.com>
Subject: 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);


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