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

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Message-ID: <20090519075715.GC4210@ff.dom.local>
Date:	Tue, 19 May 2009 07:57:15 +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 07:42:47AM +0000, Jarek Poplawski wrote:
> 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.

I might have missed your point here, but would it be so costly to do
these shifts separately here?

Jarek P.
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