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

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Date:   Tue, 26 Feb 2019 19:20:50 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>, tglx@...utronix.de,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86, retpolines: raise limit for generating indirect
 calls from switch-case

On Fri, 22 Feb 2019 at 08:32, Jesper Dangaard Brouer <brouer@...hat.com> wrote:
>
> On Thu, 21 Feb 2019 23:19:41 +0100
> Daniel Borkmann <daniel@...earbox.net> wrote:
>
> > Recent work on XDP from Björn 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 Björn 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@...hat.com>
>
> 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: Björn Töpel <bjorn.topel@...el.com>

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