Re: [RFC 2/2] xdp: Delegate fast path return decision to page_pool

[Dragos Tatulea]

On 02.12.25 15:00, Jesper Dangaard Brouer wrote:
> On 01/12/2025 11.12, Dragos Tatulea wrote:
>>
>>
>> [...]
>>>> And then you can run thus command:
>>>>   sudo ./xdp-bench redirect-map --load-egress mlx5p1 mlx5p1
>>>>
>>> Ah, yes! I was ignorant about the egress part of the program.
>>> That did the trick. The drop happens before reaching the tx
>>> queue of the second netdev and the mentioned code in devmem.c
>>> is reached.
>>>
>>> Sender is xdp-trafficgen with 3 threads pushing enough on one RX queue
>>> to saturate the CPU.
>>>
>>> Here's what I got:
>>>
>>> * before:
>>>
>>> eth2->eth3             16,153,328 rx/s         16,153,329 err,drop/s            0 xmit/s
>>>    xmit eth2->eth3               0 xmit/s       16,153,329 drop/s                0 drv_err/s         16.00 bulk-avg
>>> eth2->eth3             16,152,538 rx/s         16,152,546 err,drop/s            0 xmit/s
>>>    xmit eth2->eth3               0 xmit/s       16,152,546 drop/s                0 drv_err/s         16.00 bulk-avg
>>> eth2->eth3             16,156,331 rx/s         16,156,337 err,drop/s            0 xmit/s
>>>    xmit eth2->eth3               0 xmit/s       16,156,337 drop/s                0 drv_err/s         16.00 bulk-avg
>>>
>>> * after:
>>>
>>> eth2->eth3             16,105,461 rx/s         16,105,469 err,drop/s            0 xmit/s
>>>    xmit eth2->eth3               0 xmit/s       16,105,469 drop/s                0 drv_err/s         16.00 bulk-avg
>>> eth2->eth3             16,119,550 rx/s         16,119,541 err,drop/s            0 xmit/s
>>>    xmit eth2->eth3               0 xmit/s       16,119,541 drop/s                0 drv_err/s         16.00 bulk-avg
>>> eth2->eth3             16,092,145 rx/s         16,092,154 err,drop/s            0 xmit/s
>>>    xmit eth2->eth3               0 xmit/s       16,092,154 drop/s                0 drv_err/s         16.00 bulk-avg
>>>
>>> So slightly worse... I don't fully trust the measurements though as I
>>> saw the inverse situation in other tests as well: higher rate after the
>>> patch.
> 
> Remember that you are also removing some code (the
> xdp_set_return_frame_no_direct and xdp_clear_return_frame_no_direct).
> Thus, I was actually hoping we would see a higher rate after the patch.
> This is why I wanted to see this XDP-redirect test, instead of the
> page_pool micro-benchmark.
> 
Right. This was mentioned in the initial message as well. I was also
hoping to see an improvement...

> 
>> I had a chance to re-run this on a more stable system and the conclusion
>> is the same. Performance is ~2 % worse:
>>
>> * before:
>> eth2->eth3        13,746,431 rx/s   13,746,471 err,drop/s 0 xmit/s
>>    xmit eth2->eth3          0 xmit/s 13,746,471 drop/s     0 drv_err/s 16.00 bulk-avg
>>
>> * after:
>> eth2->eth3        13,437,277 rx/s   13,437,259 err,drop/s 0 xmit/s
>>    xmit eth2->eth3          0 xmit/s 13,437,259 drop/s     0 drv_err/s 16.00 bulk-avg
>>
>> After this experiment it doesn't seem like this direction is worth
>> proceeding with... I was more optimistic at the start.
> 
> I do think it is worth proceeding.  I will claim that your PPS results
> are basically the same. Converting PPS number to nanosec per packet:
> 
>  13,746,471 = (1/13746471*10^9) = 72.74 nanosec
>  13,437,259 = (1/13437259*10^9) = 74.42 nanosec
>  Difference is  = (74.42-72.75) =  1.67 nanosec
> 
> In my experience it is very hard to find a system stable enough to
> measure a 2 nanosec difference. As you also note you had to spend effort
> finding a stable system.  Thus, I claim your results show no noticeable
> performance impact.
> 
Oh yes, converting to ns does bring a different perspective...

> My only concern (based on your perf symbols) is that you might not be
> testing the right/expected code path.  If mlx5 is running with a
> page_pool memory mode that have elevated refcnf on the page, then we
> will not be exercising the slower page_pool ptr_ring return path as much
> as expected.  I guess, I have to do this experiment in my own testlab on
> other NIC drivers that doesn't use elevated refcnt as default.
> 
This part I don't get. I thought that the point was to measure the impact of
the change on fastest path: recycle to cache.

Are you saying that you would like to see the impact on the slowest path
as well? Or would you like to see the impact for a mix of the two? Maybe
mlx5 can be hacked into this mode for benchmarking. But not sure I understand
your usecase.

> 
>>>>> Toke (and I) will appreciate if you added code for this to xdp-bench.
>>>> Supporting a --program-mode like 'redirect-cpu' does.
>>>>
>>>>
>>> Ok. I will add it.
>>>
>> Added it here:
>> https://github.com/xdp-project/xdp-tools/pull/532
>>
> 
> Thanks, I'll take a look, and I'm sure Toke have opinions on the cmdline
> options and the missing man-page update.
> 
Oh forgot about man-page. Will wait for his PR comments.

Thanks,
Dragos