netdev - Re: [PATCH v2] Proportional Rate Reduction for TCP.

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Message-ID: <alpine.DEB.2.00.1108201534590.24318@melkinpaasi.cs.helsinki.fi>
Date:	Sat, 20 Aug 2011 15:41:36 +0300 (EEST)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Nandita Dukkipati <nanditad@...gle.com>
cc:	"David S. Miller" <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>,
	Matt Mathis <mattmathis@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH v2] Proportional Rate Reduction for TCP.

On Fri, 19 Aug 2011, Nandita Dukkipati wrote:

> Forgot to turn off gmail's rich formatting, so re-sending to the list.
> 
> On Fri, Aug 19, 2011 at 3:25 AM, Ilpo Järvinen
> <ilpo.jarvinen@...sinki.fi> wrote:
> >
> > On Fri, 19 Aug 2011, Nandita Dukkipati wrote:
> >
> > > +static void tcp_update_cwnd_in_recovery(struct sock *sk, int newly_acked_sacked,
> > > +                                     int fast_rexmit, int flag)
> > > +{
> > > +     struct tcp_sock *tp = tcp_sk(sk);
> > > +     int sndcnt = 0;
> > > +     int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);
> > > +
> > > +     if (tcp_packets_in_flight(tp) > tp->snd_ssthresh) {
> > > +             if (WARN_ON(!tp->prior_cwnd))
> > > +                     tp->prior_cwnd = 1;
> >
> > This should still be made larger to avoid problems if it ever will be
> > needed.
> 
> I am letting the value remain at 1, mainly because this is the valid
> lowest non-zero value for snd_cwnd to take on. The main purpose of
> this code is to catch any lurking bug outside of PRR which results in
> an undesirable divide by 0 in PRR. I would like to fix that bug if I
> find this code is executed.

NACK, until this value is at least 2 * tp->snd_ssthresh. Or alternatively 
the fallback is removed so that we DBZ and do not end up wrecking the 
network.

Other than that I'm ok with the patch (assuming the branches I brought
up earlier is ok for everybody else).


-- 
 i.