Re: [RFC PATCH v2] bridge: make it possible for packets to traverse the bridge without hitting netfilter

[David Woodhouse <dwmw2@infradead.org>]

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On Fri, 2015-03-06 at 17:37 +0100, Florian Westphal wrote:
> 
> > > I did performance measurements in the following way:
> > > 
> > > Removed those pieces of the packet pipeline that I don't necessarily
> > > need one-by-one.  Then measured their effect on small packet
> > > performance.
> > > 
> > > This was the only part that produced considerable effect.
> > > 
> > > The pure speculation was about why the effect is more than 15%
> > > increase in packet throughput, although the code path avoided
> > > contains way less code than 15% of the packet pipeline.  It seems,
> > > Felix Fietkau profiled similar changes, and found my guess well
> > > founded.
> > > 
> > > Now could anybody explain me what else is wrong with my patch?
> > 
> > We have to come up with a more generic solution for this.
> 
> Jiri Benc suggested to allowing to attach netfilter hooks e.g. via tc
> action, maybe that would be an option worth investigating.
> 
> Then you could for instance add filtering rules only to the bridge port
> that needs it.
> 
> > These sysfs tweaks you're proposing look to me like an obscure way to
> > tune this.
> 
> I agree, adding more tunables isn't all that helpful, in the past this
> only helped to prolong the problem.

How feasible would it be to make it completely dynamic?

A given hook could automatically disable itself (for a given device) if
the result of running it the first time was *tautologically* false for
that device (i.e. regardless of the packet itself, or anything else).

The hook would need to be automatically re-enabled if the rule chain
ever changes (and might subsequently disable itself again).

Is that something that's worth exploring for the general case?

Eschewing a 15% speedup on the basis that "well, even though we've had
three of these already for a decade, we're worried that adding a fourth
might open the floodgates to further patches" does seem a little odd to
me, FWIW.


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