Re: [PATCH bpf-next v3 8/8] bpf: add documentation for eBPF helpers (58-64)

[Quentin Monnet <quentin.monnet@netronome.com>]

2018-04-18 17:43 UTC+0200 ~ Jesper Dangaard Brouer <brouer@redhat.com>
> On Wed, 18 Apr 2018 15:09:41 +0100
> Quentin Monnet <quentin.monnet@netronome.com> wrote:
> 
>> 2018-04-18 15:34 UTC+0200 ~ Jesper Dangaard Brouer <brouer@redhat.com>
>>> On Tue, 17 Apr 2018 15:34:38 +0100
>>> Quentin Monnet <quentin.monnet@netronome.com> wrote:
>>>   
>>>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>>>> index 350459c583de..3d329538498f 100644
>>>> --- a/include/uapi/linux/bpf.h
>>>> +++ b/include/uapi/linux/bpf.h
>>>> @@ -1276,6 +1276,50 @@ union bpf_attr {
>>>>   * 	Return
>>>>   * 		0 on success, or a negative error in case of failure.
>>>>   *
>>>> + * int bpf_redirect_map(struct bpf_map *map, u32 key, u64 flags)
>>>> + * 	Description
>>>> + * 		Redirect the packet to the endpoint referenced by *map* at
>>>> + * 		index *key*. Depending on its type, his *map* can contain  
>>>                                                     ^^^
>>>
>>> "his" -> "this"  
>>
>> Thanks!
>>
>>>> + * 		references to net devices (for forwarding packets through other
>>>> + * 		ports), or to CPUs (for redirecting XDP frames to another CPU;
>>>> + * 		but this is only implemented for native XDP (with driver
>>>> + * 		support) as of this writing).
>>>> + *
>>>> + * 		All values for *flags* are reserved for future usage, and must
>>>> + * 		be left at zero.
>>>> + * 	Return
>>>> + * 		**XDP_REDIRECT** on success, or **XDP_ABORT** on error.
>>>> + *  
>>>
>>> "XDP_ABORT" -> "XDP_ABORTED"  
>>
>> Ouch. And I did the same for bpf_redirect(). Thanks for the catch.
>>
>>>
>>> I don't know if it's worth mentioning in the doc/man-page; that for XDP
>>> using bpf_redirect_map() is a HUGE performance advantage, compared to
>>> the bpf_redirect() call ?  
>>
>> It seems worth to me. How would you simply explain the reason for this
>> difference?
> 
> The basic reason is "bulking effect", as devmap avoids the NIC
> tailptr/doorbell update on every packet... how to write that in a doc
> format?
> 
> I wrote about why XDP_REDIRECT with maps are smart here:
>  http://vger.kernel.org/netconf2017_files/XDP_devel_update_NetConf2017_Seoul.pdf
> 
> Using maps for redirect, hopefully makes XDP_REDIRECT the last driver
> XDP action code we need.  As new types of redirect can be introduced
> without driver changes. See that AF_XDP also uses a map.
> 
> It is more subtle, but maps also function as a sorting step. Imagine
> your XDP program need to redirect out different interfaces (or CPUs in
> cpumap case), and packets arrive intermixed.  Packets get sorted into
> the different map indexes, and the xdp_do_flush_map() will trigger the
> flush operation.
> 
> 
> Happened to have an i40e NIC benchmark setup, and ran a single flow pktgen test.
> 
> Results with 'xdp_redirect_map'
>  13589297 pps (13,589,297) 
> 
> Results with 'xdp_redirect' NOT using devmap:
>   7567575 pps (7,567,575)
> 
> Just to point out the performance benefit of devmap...


Thanks for those details! This is an impressive change in performance
indeed.

I think I will just keep it simple for the documentation. I will add the
following for bpf_redirect_map():

    When used to redirect packets to net devices, this helper
    provides a high performance increase over **bpf_redirect**\ ().
    This is due to various implementation details of the underlying
    mechanisms, one of which is the fact that **bpf_redirect_map**\ ()
    tries to send packet as a "bulk" to the device.

And also append the following to bpf_redirect():

    The same effect can be attained with the more generic
    **bpf_redirect_map**\ (), which requires specific maps
    to be used but offers better performance.

Best,
Quentin