Re: [PATCH V3 net-next] net: fec: add XDP_TX feature support

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

On 03/08/2023 13.18, Wei Fang wrote:
>> On 03/08/2023 05.58, Wei Fang wrote:
>>>>>                    } else {
>>>>> -                 xdp_return_frame(xdpf);
>>>>> +                 xdp_return_frame_rx_napi(xdpf);
>>>> If you implement Jesper's syncing suggestions, I think you can use
>>>>
>>>>     page_pool_put_page(pool, page, 0, true);
>> To Jakub, using 0 here you are trying to bypass the DMA-sync (which is valid
>> as driver knows XDP_TX have already done the sync).
>> The code will still call into DMA-sync calls with zero as size, so wonder if we
>> should detect size zero and skip that call?
>> (I mean is this something page_pool should support.)
>>
[...]
>>
>>
>>>> for XDP_TX here to avoid the DMA sync on page recycle.
>>> I tried Jasper's syncing suggestion and used page_pool_put_page() to
>>> recycle pages, but the results does not seem to improve the
>>> performance of XDP_TX,
>> The optimization will only have effect on those devices which have
>> dev->dma_coherent=false else DMA function [1] (e.g.
>> dma_direct_sync_single_for_device) will skip the sync calls.
>>
>>  [1] https://elixir.bootlin.com/linux/v6.5-rc4/source/kernel/dma/direct.h#L63
>>
>> (Cc. Andrew Lunn)
>> Does any of the imx generations have dma-noncoherent memory?
>>
>> And does any of these use the fec NIC driver?
>>
>>> it even degrades the speed.
>> 
>> Could be low runs simply be a variation between your test runs?
>>
> Maybe, I just tested once before. So I test several times again, the
> results of the two methods do not seem to be much different so far,
> both about 255000 pkt/s.
> 
>> The specific device (imx8mpevk) this was tested on, clearly have
>> dma_coherent=true, or else we would have seen a difference.
>> But the code change should not have any overhead for the
>> dma_coherent=true case, the only extra overhead is the extra empty DMA
>> sync call with size zero (as discussed in top).
>>
> The FEC of i.MX8MP-EVK has dma_coherent=false, and as I mentioned
> above, I did not see an obvious difference in the performance. :(

That is surprising - given the results.

(see below, lack of perf/diff might be caused by Ethernet flow-control).

> 
>>> The result of the current modification.
>>> root@imx8mpevk:~# ./xdp2 eth0
>>> proto 17:     260180 pkt/s
>>
>> These results are*significantly*  better than reported in patch-1.
>> What happened?!?
>>
> The test environment is slightly different, in patch-1, the FEC port was
> directly connected to the port of another board. But in the latest test,
> the ports of the two boards were connected to a switch, so the ports of
> the two boards are not directly connected.
> 

Hmm, I've seen this kind of perf behavior of direct-connected or via
switch before. The mistake I made was, that I had not disabled Ethernet
flow-control.  The xdp2 XDP_TX program will swap the mac addresses, and
send the packet back to the packet generator (running pktgen), which
will get overloaded itself and starts sending Ethernet flow-control
pause frames.

Command line to disable:
  # ethtool -A eth0 rx off tx off

Can I ask/get you to make sure that Ethernet flow-control is disabled
(on both generator and DUT (to be on safe-side)) and run the test again?

--Jesper

>> e.g.
>>    root@imx8mpevk:~# ./xdp2 eth0
>>    proto 17:     135817 pkt/s
>>    proto 17:     142776 pkt/s
>>
>>> proto 17:     260373 pkt/s
>>> proto 17:     260363 pkt/s
>>> proto 17:     259036 pkt/s
[...]
>>>
>>> After using the sync suggestion, the result shows as follow.
>>> root@imx8mpevk:~# ./xdp2 eth0
>>> proto 17:     255956 pkt/s
>>> proto 17:     255841 pkt/s
>>> proto 17:     255835 pkt/s