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

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Date:	Thu, 5 Jan 2012 19:30:56 +0000
From:	"Yalagandula, Praveen" <praveen.yalagandula@...com>
To:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
CC:	"Yalagandula, Praveen" <praveen.yalagandula@...com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: bug in qavg calculation in include/net/red.h (resending as
 plain-text email)

(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.

Thanks,
Praveen Yalagandula
Senior Researcher, HP Labs, Palo Alto, CA

-------------------------------------

static inline unsigned long red_calc_qavg_no_idle_time(struct red_parms *p, unsigned int backlog)
{
     /*
     * NOTE: p->qavg is fixed point number with point at Wlog.
     * The formula below is equvalent to floating point
     * version:
     *
     *   qavg = qavg*(1-W) + backlog*W;
     *
     * --ANK (980924)
     */
     return p->qavg + (backlog - (p->qavg >> p->Wlog));
}
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