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

Jesper Dangaard Brouer
2023-08-04T12:36:45Z

On 04/08/2023 05.06, Wei Fang wrote:
>>> 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?
>>
> The flow-control was not disabled before, so according to your suggestion,
> I disable the flow-control on the both boards and run the test again, the
> performance is slightly improved, but still can not see a clear difference
> between the two methods. Below are the results.

Something else must be stalling the CPU.
When looking at fec_main.c code, I noticed that
fec_enet_txq_xmit_frame() will do a MMIO write for every xdp_frame (to
trigger transmit start), which I believe will stall the CPU.
The ndo_xdp_xmit/fec_enet_xdp_xmit does bulking, and should be the
function that does the MMIO write to trigger transmit start.

$ git diff
diff --git a/drivers/net/ethernet/freescale/fec_main.c
b/drivers/net/ethernet/freescale/fec_main.c
index 03ac7690b5c4..57a6a3899b80 100644
--- a/drivers/net/ethernet/freescale/fec_main.c
+++ b/drivers/net/ethernet/freescale/fec_main.c
@@ -3849,9 +3849,6 @@ static int fec_enet_txq_xmit_frame(struct
fec_enet_private *fep,

txq->bd.cur = bdp;

- /* Trigger transmission start */
- writel(0, txq->bd.reg_desc_active);
-
return 0;
}

@@ -3880,6 +3877,9 @@ static int fec_enet_xdp_xmit(struct net_device *dev,
sent_frames++;
}

+ /* Trigger transmission start */
+ writel(0, txq->bd.reg_desc_active);
+
__netif_tx_unlock(nq);

return sent_frames;

> Result: use "sync_dma_len" method
> root@imx8mpevk:~# ./xdp2 eth0

The xdp2 (and xdp1) program(s) have a performance issue
(due to using

Can I ask you to test using xdp_rxq_info, like:

sudo ./xdp_rxq_info --dev mlx5p1 --action XDP_TX

> proto 17: 258886 pkt/s
> proto 17: 258879 pkt/s

If you provide numbers for xdp_redirect, then we could better evaluate
if changing the lock per xdp_frame, for XDP_TX also, is worth it.

And also find out of moving the MMIO write have any effect.

I also noticed driver does a MMIO write (on rxq) for every RX-packet in
fec_enet_rx_queue() napi-poll loop. This also looks like a potential
performance stall.

--Jesper