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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>,
	"Björn Töpel" <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
Date: Fri, 22 Feb 2019 08:31:31 +0100	[thread overview]
Message-ID: <20190222083131.5a141e23@carbon> (raw)
In-Reply-To: <20190221221941.29358-1-daniel@iogearbox.net>

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

  parent reply	other threads:[~2019-02-22  7:31 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-21 22:19 [PATCH] x86, retpolines: raise limit for generating indirect calls from switch-case Daniel Borkmann
2019-02-21 22:27 ` Linus Torvalds
2019-02-21 22:56   ` Daniel Borkmann
2019-02-22  7:31 ` Jesper Dangaard Brouer [this message]
2019-02-26 18:20   ` Björn Töpel
2019-02-28 11:12 ` [tip:x86/build] x86, retpolines: Raise " tip-bot for Daniel Borkmann
2019-02-28 11:27   ` David Woodhouse
2019-02-28 12:53     ` H.J. Lu
2019-02-28 16:18       ` Daniel Borkmann
2019-02-28 16:25         ` H.J. Lu
2019-02-28 17:58           ` Daniel Borkmann
2019-02-28 18:09             ` H.J. Lu
2019-02-28 18:13               ` Daniel Borkmann

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