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

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Date:   Fri, 22 Feb 2019 08:31:31 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org,
        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>,
        brouer@...hat.com
Subject: Re: [PATCH] x86, retpolines: raise limit for generating indirect
 calls from switch-case

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