netdev - Re: [PATCH] pkt_sched: gen_estimator: use 64 bits intermediate counters for bps

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Message-ID: <20090519074247.GB4210@ff.dom.local>
Date:	Tue, 19 May 2009 07:42:47 +0000
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	David Miller <davem@...emloft.net>, vexwek@...il.com,
	netdev@...r.kernel.org, kaber@...sh.net, devik@....cz
Subject: Re: [PATCH] pkt_sched: gen_estimator: use 64 bits intermediate
	counters for bps

On Tue, May 19, 2009 at 09:31:36AM +0200, Eric Dumazet wrote:
> Jarek Poplawski a écrit :
> > On Tue, May 19, 2009 at 01:59:55AM +0200, Eric Dumazet wrote:
> > ...
> >> diff --git a/net/core/gen_estimator.c b/net/core/gen_estimator.c
> > ...
> >> -		e->avbps += ((long)rate - (long)e->avbps) >> e->ewma_log;
> >> +		e->avbps += ((s64)(brate - e->avbps)) >> e->ewma_log;
> > 
> > Btw., I'm a bit concerned about the syntax here: isn't such shifting
> > of signed ints implementation dependant?
> > 
> 
> You are right Jarek, I very often forget to never ever use signed quantities
> at all ! (But also note original code has same undefined behavior)

Sure, I've meant the original code including 5 lines below.

> Apparently gcc does the *right* thing on x86_32, but we probably want something
> stronger here. I could not find gcc documentation statement on right shifts of 
> negative values.

I guess gcc and most of others do this "right"; but it looks
"unkosher" anyway.

Jarek P.
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