Subject: Re: [PATCH net-next v3 5/6] tsnep: Add XDP socket zero-copy RX support

From: Maciej Fijalkowski <[email-masked]>
Date: Thu, 20 Apr 2023 21:46:33 +0200

On Tue, Apr 18, 2023 at 09:04:58PM +0200, Gerhard Engleder wrote:
> Add support for XSK zero-copy to RX path. The setup of the XSK pool can
> be done at runtime. If the netdev is running, then the queue must be
> disabled and enabled during reconfiguration. This can be done easily
> with functions introduced in previous commits.
> 
> A more important property is that, if the netdev is running, then the
> setup of the XSK pool shall not stop the netdev in case of errors. A
> broken netdev after a failed XSK pool setup is bad behavior. Therefore,
> the allocation and setup of resources during XSK pool setup is done only
> before any queue is disabled. Additionally, freeing and later allocation
> of resources is eliminated in some cases. Page pool entries are kept for
> later use. Two memory models are registered in parallel. As a result,
> the XSK pool setup cannot fail during queue reconfiguration.
> 
> In contrast to other drivers, XSK pool setup and XDP BPF program setup
> are separate actions. XSK pool setup can be done without any XDP BPF
> program. The XDP BPF program can be added, removed or changed without
> any reconfiguration of the XSK pool.
> 
> Test results with A53 1.2GHz:
> 
> xdpsock rxdrop copy mode:
>                    pps            pkts           1.00
> rx                 856,054        10,625,775
> Two CPUs with both 100% utilization.
> 
> xdpsock rxdrop zero-copy mode:
>                    pps            pkts           1.00
> rx                 889,388        4,615,284
> Two CPUs with 100% and 20% utilization.
> 
> xdpsock l2fwd copy mode:
>                    pps            pkts           1.00
> rx                 248,985        7,315,885
> tx                 248,921        7,315,885
> Two CPUs with 100% and 10% utilization.
> 
> xdpsock l2fwd zero-copy mode:
>                    pps            pkts           1.00
> rx                 254,735        3,039,456
> tx                 254,735        3,039,456
> Two CPUs with 100% and 4% utilization.

Thanks for sharing the numbers. This is for 64 byte frames?

> 
> Packet rate increases and CPU utilization is reduced in both cases.
> 100% CPU load seems to the base load. This load is consumed by ksoftirqd
> just for dropping the generated packets without xdpsock running.
> 
> Using batch API reduced CPU utilization slightly, but measurements are
> not stable enough to provide meaningful numbers.
> 
> Signed-off-by: Gerhard Engleder <[email-masked]>
> ---
>  drivers/net/ethernet/engleder/tsnep.h      |  13 +-
>  drivers/net/ethernet/engleder/tsnep_main.c | 494 ++++++++++++++++++++-
>  drivers/net/ethernet/engleder/tsnep_xdp.c  |  66 +++
>  3 files changed, 558 insertions(+), 15 deletions(-)
> 

(...)

>  static const struct net_device_ops tsnep_netdev_ops = {
>  	.ndo_open = tsnep_netdev_open,
>  	.ndo_stop = tsnep_netdev_close,
> @@ -1713,6 +2177,7 @@ static const struct net_device_ops tsnep_netdev_ops = {
>  	.ndo_setup_tc = tsnep_tc_setup,
>  	.ndo_bpf = tsnep_netdev_bpf,
>  	.ndo_xdp_xmit = tsnep_netdev_xdp_xmit,
> +	.ndo_xsk_wakeup = tsnep_netdev_xsk_wakeup,
>  };
>  
>  static int tsnep_mac_init(struct tsnep_adapter *adapter)
> @@ -1973,7 +2438,8 @@ static int tsnep_probe(struct platform_device *pdev)
>  
>  	netdev->xdp_features = NETDEV_XDP_ACT_BASIC | NETDEV_XDP_ACT_REDIRECT |
>  			       NETDEV_XDP_ACT_NDO_XMIT |
> -			       NETDEV_XDP_ACT_NDO_XMIT_SG;
> +			       NETDEV_XDP_ACT_NDO_XMIT_SG |
> +			       NETDEV_XDP_ACT_XSK_ZEROCOPY;

In theory enabling this feature here before implementing Tx ZC can expose
you to some broken behavior, so just for the sake of completeness, i would
move this to Tx ZC patch.