netdev - Re: [net-next 01/12] i40e: replace switch-statement to speed-up retpoline-enabled builds

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Date:   Thu, 02 May 2019 13:57:34 -0700
From:   Jeff Kirsher <jeffrey.t.kirsher@...el.com>
To:     Björn Töpel <bjorn.topel@...el.com>,
        Daniel Borkmann <daniel@...earbox.net>, davem@...emloft.net
Cc:     netdev@...r.kernel.org, nhorman@...hat.com, sassmann@...hat.com,
        Andrew Bowers <andrewx.bowers@...el.com>
Subject: Re: [net-next 01/12] i40e: replace switch-statement to speed-up
 retpoline-enabled builds

On Thu, 2019-05-02 at 22:56 +0200, Björn Töpel wrote:
> On 2019-05-02 22:40, Jeff Kirsher wrote:
> > On Thu, 2019-05-02 at 22:29 +0200, Björn Töpel wrote:
> > > On 2019-05-02 16:47, Daniel Borkmann wrote:
> > > > On 04/29/2019 09:16 PM, Jeff Kirsher wrote:
> > > > > From: Björn Töpel <bjorn.topel@...el.com>
> > > > > 
> > > > > GCC will generate jump tables for switch-statements with more
> > > > > than 5
> > > > > case statements. An entry into the jump table is an indirect
> > > > > call,
> > > > > which means that for CONFIG_RETPOLINE builds, this is rather
> > > > > expensive.
> > > > > 
> > > > > This commit replaces the switch-statement that acts on the XDP
> > > > > program
> > > > > result with an if-clause.
> > > > > 
> > > > > The if-clause was also refactored into a common function that can
> > > > > be
> > > > > used by AF_XDP zero-copy and non-zero-copy code.
> > > > 
> > > > Isn't it fixed upstream by now already (also in gcc)?
> > > > 
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ce02ef06fcf7a399a6276adb83f37373d10cbbe1
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a9d57ef15cbe327fe54416dd194ee0ea66ae53a4
> > > > 
> > > 
> > > Hmm, given that Daniel's work is upstream, this patch doesn't really
> > > make sense any more. OTOH it can stay in the series, and be cleaned
> > > up
> > > later.
> > > 
> > > I'll leave it for you to decide, Jeff!
> > 
> > I am already making revisions to the series due to another patch, so if
> > these changes are no longer needed to improve performance in RETPOLINE
> > builds, then lets drop it.
> > 
> > Björn, can you confirm that with or without these changes, XDP
> > performance
> > stays the same for RETPOLINE builds?
> > 
> 
> Confirmed (on i40e using xdp1 and xdpsock samples); Same performance
> with/without this patch.
> 
> IOW, please drop this from your next spin.

Ok, dropped.

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