netdev - RE: bug in qavg calculation in include/net/red.h (resending as plain-text email)

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Message-ID: <67E2BF19E6D6A749A7680AEF496EB37C39D92B263A@GVW0547EXC.americas.hpqcorp.net>
Date:	Thu, 5 Jan 2012 21:29:38 +0000
From:	"Yalagandula, Praveen" <praveen.yalagandula@...com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: bug in qavg calculation in include/net/red.h (resending as
 plain-text email)

Thanks for the prompt response. I understand it better now.

Cheers,
Praveen

> -----Original Message-----
> From: Eric Dumazet [mailto:eric.dumazet@...il.com]
> Sent: Thursday, January 05, 2012 1:20 PM
> To: Yalagandula, Praveen
> Cc: Alexey Kuznetsov; netdev@...r.kernel.org
> Subject: Re: bug in qavg calculation in include/net/red.h (resending as
> plain-text email)
> 
> Le jeudi 05 janvier 2012 à 21:13 +0100, Eric Dumazet a écrit :
> > Le jeudi 05 janvier 2012 à 19:30 +0000, Yalagandula, Praveen a écrit
> :
> > > (Resending as plain text)
> > >
> > > Dear Alexey,
> > >
> > > The code (see below) for computing the queue average for RED
> > > implementation in Linux kernel has a bug. The return line should be
> > > "return p->qavg + ((backlog - p->qavg) >> p->Wlog);".
> > >
> > > qavg = qavg*(1-W) + backlog*W
> > >
> > >      = qavg + (backlog-qavg)*W
> > >
> > > The current buggy code applies right shift to only the previous
> qavg,
> > > where as it should be applied to (backlog - qavg). Hope you can get
> > > this bug corrected.
> >
> > Code is correct.
> >
> > Please read the comment, it really explains the thing.
> >
> 
> I have more time to explain what's going on :
> 
> On input, "unsigned int backlog" is not yet scaled.
> But qavg _is_ scaled by p->Wlog.
> 
> So we should first scale "backlog" by Wlog bits to the left :
> 
> backlog_scaled = backlog << p->Wlog;
> 
> Then we do our computation as you stated :
> 
> return p->qavg + ((backlog_scaled - p->qavg) >> p->Wlog);
> 
> Its equivalent to current code, but using a single shift to the right.
> 
> return p->qavg + (backlog - (p->qavg >> p->Wlog));
> 
> 
> Hope this helps.
>