[xdp-hints] Re: [PATCH RFC bpf-next 32/52] bpf, cpumap: switch to GRO from netif_receive_skb_list()

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 13/08/2024 16.54, Toke Høiland-Jørgensen wrote:
> Alexander Lobakin <aleksander.lobakin@intel.com> writes:
> 
>> From: Alexander Lobakin <aleksander.lobakin@intel.com>
>> Date: Thu, 8 Aug 2024 13:57:00 +0200
>>
>>> From: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
>>> Date: Thu, 8 Aug 2024 06:54:06 +0200
>>>
>>>>> Hi Alexander,
>>>>>
>>>>> On Tue, Jun 28, 2022, at 12:47 PM, Alexander Lobakin wrote:
>>>>>> cpumap has its own BH context based on kthread. It has a sane batch
>>>>>> size of 8 frames per one cycle.
>>>>>> GRO can be used on its own, adjust cpumap calls to the
>>>>>> upper stack to use GRO API instead of netif_receive_skb_list() which
>>>>>> processes skbs by batches, but doesn't involve GRO layer at all.
>>>>>> It is most beneficial when a NIC which frame come from is XDP
>>>>>> generic metadata-enabled, but in plenty of tests GRO performs better
>>>>>> than listed receiving even given that it has to calculate full frame
>>>>>> checksums on CPU.
>>>>>> As GRO passes the skbs to the upper stack in the batches of
>>>>>> @gro_normal_batch, i.e. 8 by default, and @skb->dev point to the
>>>>>> device where the frame comes from, it is enough to disable GRO
>>>>>> netdev feature on it to completely restore the original behaviour:
>>>>>> untouched frames will be being bulked and passed to the upper stack
>>>>>> by 8, as it was with netif_receive_skb_list().
>>>>>>
>>>>>> Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
>>>>>> ---
>>>>>>   kernel/bpf/cpumap.c | 43 ++++++++++++++++++++++++++++++++++++++-----
>>>>>>   1 file changed, 38 insertions(+), 5 deletions(-)
>>>>>>
>>>>>
>>>>> AFAICT the cpumap + GRO is a good standalone improvement. I think
>>>>> cpumap is still missing this.
>>>
>>> The only concern for having GRO in cpumap without metadata from the NIC
>>> descriptor was that when the checksum status is missing, GRO calculates
>>> the checksum on CPU, which is not really fast.
>>> But I remember sometimes GRO was faster despite that.
>>>
>>>>>
>>>>> I have a production use case for this now. We want to do some intelligent
>>>>> RX steering and I think GRO would help over list-ified receive in some cases.
>>>>> We would prefer steer in HW (and thus get existing GRO support) but not all
>>>>> our NICs support it. So we need a software fallback.
>>>>>
>>>>> Are you still interested in merging the cpumap + GRO patches?
>>>
>>> For sure I can revive this part. I was planning to get back to this
>>> branch and pick patches which were not related to XDP hints and send
>>> them separately.
>>>
>>>>
>>>> Hi Daniel and Alex,
>>>>
>>>> Recently I worked on a PoC to add GRO support to cpumap codebase:
>>>> - https://github.com/LorenzoBianconi/bpf-next/commit/a4b8264d5000ecf016da5a2dd9ac302deaf38b3e
>>>>    Here I added GRO support to cpumap through gro-cells.
>>>> - https://github.com/LorenzoBianconi/bpf-next/commit/da6cb32a4674aa72401c7414c9a8a0775ef41a55
>>>>    Here I added GRO support to cpumap trough napi-threaded APIs (with a some
>>>>    changes to them).
>>>
>>> Hmm, when I was testing it, adding a whole NAPI to cpumap was sorta
>>> overkill, that's why I separated GRO structure from &napi_struct.
>>>
>>> Let me maybe find some free time, I would then test all 3 solutions
>>> (mine, gro_cells, threaded NAPI) and pick/send the best?
>>>
>>>>
>>>> Please note I have not run any performance tests so far, just verified it does
>>>> not crash (I was planning to resume this work soon). Please let me know if it
>>>> works for you.
>>
>> I did tests on both threaded NAPI for cpumap and my old implementation
>> with a traffic generator and I have the following (in Kpps):
>>

What kind of traffic is the traffic generator sending?

E.g. is this a type of traffic that gets GRO aggregated?

>>              direct Rx    direct GRO    cpumap    cpumap GRO
>> baseline    2900         5800          2700      2700 (N/A)
>> threaded                               2300      4000
>> old GRO                                2300      4000
>>

Nice results. Just to confirm, the units are in Kpps.


>> IOW,
>>
>> 1. There are no differences in perf between Lorenzo's threaded NAPI
>>     GRO implementation and my old implementation, but Lorenzo's is also
>>     a very nice cleanup as it switches cpumap to threaded NAPI completely
>>     and the final diffstat even removes more lines than adds, while mine
>>     adds a bunch of lines and refactors a couple hundred, so I'd go with
>>     his variant.
>>
>> 2. After switching to NAPI, the performance without GRO decreases (2.3
>>     Mpps vs 2.7 Mpps), but after enabling GRO the perf increases hugely
>>     (4 Mpps vs 2.7 Mpps) even though the CPU needs to compute checksums
>>     manually.
> 
> One question for this: IIUC, the benefit of GRO varies with the traffic
> mix, depending on how much the GRO logic can actually aggregate. So did
> you test the pathological case as well (spraying packets over so many
> flows that there is basically no aggregation taking place)? Just to make
> sure we don't accidentally screw up performance in that case while
> optimising for the aggregating case :)
> 

For the GRO use-case, I think a basic TCP stream throughput test (like
netperf) should show a benefit once cpumap enable GRO, Can you confirm this?
Or does the missing hardware RX-hash and RX-checksum cause TCP GRO not
to fully work, yet?

Thanks A LOT for doing this benchmarking!
--Jesper