Re: [PATCH] x86, retpolines: raise limit for generating indirect calls from switch-case

[BjÃrn TÃpel]

On Fri, 22 Feb 2019 at 08:32, Jesper Dangaard Brouer <brouer@xxxxxxxxxx> wrote:
>
> On Thu, 21 Feb 2019 23:19:41 +0100
> Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> > Recent work on XDP from BjÃrn and Magnus additionally found that
> > manually transforming the XDP return code switch statement with
> > more than 5 cases into if-else combination would result in a
> > considerable speedup in XDP layer due to avoidance of indirect
> > calls in CONFIG_RETPOLINE enabled builds. On i40e driver with
> > XDP prog attached, a 20-26% speedup has been observed [0]. Aside
> > from XDP, there are many other places later in the networking
> > stack's critical path with similar switch-case processing. Rather
> > than fixing every XDP-enabled driver and locations in stack by
> > hand, it would be good to instead raise the limit where gcc would
> > emit expensive indirect calls from the switch under retpolines
>
> I'm very happy to see this.  Thanks to BjÃrn for finding, analyzing and
> providing hand-coded-if-else code that demonstrated the performance
> issue for XDP.  But I do think this GCC case-values-threshold param is
> a better and more generic solution to the issue we observed and
> measured in XDP land. And hopefully other parts of the network stack
> and kernel will also benefit.
>
> Acked-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
>
> Thanks for following up on this Daniel,

I definitely prefer a switch-statement over the if-else-messiness in
this context. Thanks for doing the deep-dive, Daniel!

FWIW,
Acked-by: BjÃrn TÃpel <bjorn.topel@xxxxxxxxx>

> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer