Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 22 Feb 2019 08:31:31 +0100
From:   Jesper Dangaard Brouer <brouer@redhat.com>
To:     Daniel Borkmann <daniel@iogearbox.net>
Cc:     tglx@linutronix.de, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, "David S . Miller" <davem@davemloft.net>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@intel.com>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        brouer@redhat.com
Subject: Re: [PATCH] x86, retpolines: raise limit for generating indirect
 calls from switch-case
Message-ID: <20190222083131.5a141e23@carbon>
In-Reply-To: <20190221221941.29358-1-daniel@iogearbox.net>
References: <20190221221941.29358-1-daniel@iogearbox.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, 21 Feb 2019 23:19:41 +0100
Daniel Borkmann <daniel@iogearbox.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@redhat.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