netdev - Re: [PATCH iproute2] htb: report overhead attribute

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Message-ID: <1370354311.24311.205.camel@edumazet-glaptop>
Date:	Tue, 04 Jun 2013 06:58:31 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:	Rick Jones <rick.jones2@...com>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	netdev <netdev@...r.kernel.org>,
	"russell-tcatm@...art.id.au" <russell-tcatm@...art.id.au>
Subject: Re: [PATCH iproute2] htb: report overhead attribute

On Tue, 2013-06-04 at 13:11 +0200, Jesper Dangaard Brouer wrote:
> On Mon, 03 Jun 2013 08:56:02 -0700
> Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> > On Mon, 2013-06-03 at 08:45 -0700, Rick Jones wrote:
> > 
> > > Is it (still) possible to have a negative overhead?
> > > 
> > > http://www.linksysinfo.org/index.php?threads/speedmod-with-tc-atm-qos-patch-for-adsl.31541/
> > 
> > overhead always has been unsigned in the kernel.
> > 
> > What you describe is a userland hack in tc command.
> > (or a bug)
> 
> Rick is referencing Russell Stuart's patches, where a negative overhead
> was possible.
>  http://ace-host.stuart.id.au/russell/files/tc/tc-atm/#history
> 
> But my patches got accepted into the kernel, where a negative
> overhead was not possible. In retrospect, we should have supported a
> negative overhead.
> 
> A negative overhead *is* a valid use-case, and we should work towards
> supporting this. E.g. by changing the recent added "u16 overhead" in
> struct psched_ratecfg to be "s16" (ref [1]) ?
> 
> 
> [1] commit 01cb71d2d47 (net_sched: restore "overhead xxx" handling)
>  https://git.kernel.org/cgit/linux/kernel/git/davem/net.git/commit/?id=01cb71d2d47b78354358e4bb938bb06323e17498
> 

Again, you describe something that Vimal patch didn't broke, and should
be addressed on net-next.

My concern was restoring the overhead attribute that Vimal broke, and
this one was unsigned 16bits.

Allowing a negative offset is not free, it adds a conditional test,
because (len + overhead) could be negative.

Please note that a negative offset is possible with the STAB infra.


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