netdev - Re: [PATCH RFC net-next 1/2] net: Reference bpf_redirect_info via task_struct on PREEMPT

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Date: Thu, 22 Feb 2024 11:10:44 +0100
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Toke Høiland-Jørgensen <toke@...hat.com>,
 bpf@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH RFC net-next 1/2] net: Reference bpf_redirect_info via
 task_struct on PREEMPT_RT.

On 22/02/2024 10.22, Sebastian Andrzej Siewior wrote:
> On 2024-02-20 16:32:08 [+0100], To Jesper Dangaard Brouer wrote:
>>>
>>>   Ethtool(i40e2) stat:     15028585 (  15,028,585) <= tx-0.packets /sec
>>>   Ethtool(i40e2) stat:     15028589 (  15,028,589) <= tx_packets /sec
>>
>> -t1 in ixgbe
>> Show adapter(s) (eth1) statistics (ONLY that changed!)
>> Ethtool(eth1    ) stat:    107857263 (    107,857,263) <= tx_bytes /sec
>> Ethtool(eth1    ) stat:    115047684 (    115,047,684) <= tx_bytes_nic /sec
>> Ethtool(eth1    ) stat:      1797621 (      1,797,621) <= tx_packets /sec
>> Ethtool(eth1    ) stat:      1797636 (      1,797,636) <= tx_pkts_nic /sec
>> Ethtool(eth1    ) stat:    107857263 (    107,857,263) <= tx_queue_0_bytes /sec
>> Ethtool(eth1    ) stat:      1797621 (      1,797,621) <= tx_queue_0_packets /sec
> …
>> while sending with ixgbe while running perf top on the box:
>> | Samples: 621K of event 'cycles', 4000 Hz, Event count (approx.): 49979376685 lost: 0/0 drop: 0/0
>> | Overhead  CPU  Command          Shared Object             Symbol
>> |   31.98%  000  kpktgend_0       [kernel]                  [k] xas_find
>> |    6.72%  000  kpktgend_0       [kernel]                  [k] pfn_to_dma_pte
>> |    5.63%  000  kpktgend_0       [kernel]                  [k] ixgbe_xmit_frame_ring
>> |    4.78%  000  kpktgend_0       [kernel]                  [k] dma_pte_clear_level
>> |    3.16%  000  kpktgend_0       [kernel]                  [k] __iommu_dma_unmap
> 
> I disabled the iommu and I get to

Yes, clearly IOMMU code that cause the performance issue for you.

This driver doesn't use page_pool, so I want to point out (for people
finding this post in the future) that page_pool keeps DMA mappings for
recycled frame, which should address the IOMMU overhead issue here.

> 
> Ethtool(eth1    ) stat:     14158562 (     14,158,562) <= tx_packets /sec
> Ethtool(eth1    ) stat:     14158685 (     14,158,685) <= tx_pkts_nic /sec
> 
> looks like a small improvement… It is not your 15 but close. -t2 does
> improve the situation. 

You cannot reach 15Mpps on 10Gbit/s as wirespeed for 10G is 14.88Mpps.

Congratulations, I think this 14.15 Mpps is as close to wirespeed as it
possible on your hardware.

BTW what CPU are you using?

> There is a warning from DMA mapping code but ;)

It is a warning from IOMMU code?
It usually means there is a real DMA unmap bug (which we should fix).

--Jesper