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

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Date:   Thu, 2 May 2019 22:56:23 +0200
From:   Björn Töpel <bjorn.topel@...el.com>
To:     Jeff Kirsher <jeffrey.t.kirsher@...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 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.


Thanks,
Björn