Subject: Re: [PATCH iwl-next,v4 1/1] igc: Add Tx hardware timestamp request for AF_XDP zero-copy packet

From: Vinicius Costa Gomes <[email-masked]>
Date: Mon, 25 Mar 2024 19:29:04 -0700

Song Yoong Siang <[email-masked]> writes:

> This patch adds support to per-packet Tx hardware timestamp request to
> AF_XDP zero-copy packet via XDP Tx metadata framework. Please note that
> user needs to enable Tx HW timestamp capability via igc_ioctl() with
> SIOCSHWTSTAMP cmd before sending xsk Tx hardware timestamp request.
>
> Same as implementation in RX timestamp XDP hints kfunc metadata, Timer 0
> (adjustable clock) is used in xsk Tx hardware timestamp. i225/i226 have
> four sets of timestamping registers. *skb and *xsk_tx_buffer pointers
> are used to indicate whether the timestamping register is already occupied.
>
> Furthermore, a boolean variable named xsk_pending_ts is used to hold the
> transmit completion until the tx hardware timestamp is ready. This is
> because, for i225/i226, the timestamp notification event comes some time
> after the transmit completion event. The driver will retrigger hardware irq
> to clean the packet after retrieve the tx hardware timestamp.
>
> Besides, xsk_meta is added into struct igc_tx_timestamp_request as a hook
> to the metadata location of the transmit packet. When the Tx timestamp
> interrupt is fired, the interrupt handler will copy the value of Tx hwts
> into metadata location via xsk_tx_metadata_complete().
>
> This patch is tested with tools/testing/selftests/bpf/xdp_hw_metadata
> on Intel ADL-S platform. Below are the test steps and results.
>
> Test Step 1: Run xdp_hw_metadata app
>  ./xdp_hw_metadata <iface> > /dev/shm/result.log
>
> Test Step 2: Enable Tx hardware timestamp
>  hwstamp_ctl -i <iface> -t 1 -r 1
>
> Test Step 3: Run ptp4l and phc2sys for time synchronization
>
> Test Step 4: Generate UDP packets with 1ms interval for 10s
>  trafgen --dev <iface> '{eth(da=<addr>), udp(dp=9091)}' -t 1ms -n 10000
>
> Test Step 5: Rerun Step 1-3 with 10s iperf3 as background traffic
>
> Test Step 6: Rerun Step 1-4 with 10s iperf3 as background traffic
>
> Based on iperf3 results below, the impact of holding tx completion to
> throughput is not observable.
>
> Result of last UDP packet (no. 10000) in Step 4:
> poll: 1 (0) skip=99 fail=0 redir=10000
> xsk_ring_cons__peek: 1
> 0x5640a37972d0: rx_desc[9999]->addr=f2110 addr=f2110 comp_addr=f2110 EoP
> rx_hash: 0x2049BE1D with RSS type:0x1
> HW RX-time:   1679819246792971268 (sec:1679819246.7930) delta to User RX-time sec:0.0000 (14.990 usec)
> XDP RX-time:   1679819246792981987 (sec:1679819246.7930) delta to User RX-time sec:0.0000 (4.271 usec)
> No rx_vlan_tci or rx_vlan_proto, err=-95
> 0x5640a37972d0: ping-pong with csum=ab19 (want 315b) csum_start=34 csum_offset=6
> 0x5640a37972d0: complete tx idx=9999 addr=f010
> HW TX-complete-time:   1679819246793036971 (sec:1679819246.7930) delta to User TX-complete-time sec:0.0001 (77.656 usec)
> XDP RX-time:   1679819246792981987 (sec:1679819246.7930) delta to User TX-complete-time sec:0.0001 (132.640 usec)
> HW RX-time:   1679819246792971268 (sec:1679819246.7930) delta to HW TX-complete-time sec:0.0001 (65.703 usec)
> 0x5640a37972d0: complete rx idx=10127 addr=f2110
>
> Result of iperf3 without tx hwts request in step 5:
> [ ID] Interval           Transfer     Bitrate         Retr
> [  5]   0.00-10.00  sec  2.74 GBytes  2.36 Gbits/sec    0             sender
> [  5]   0.00-10.05  sec  2.74 GBytes  2.34 Gbits/sec                  receiver
>
> Result of iperf3 running parallel with trafgen command in step 6:
> [ ID] Interval           Transfer     Bitrate         Retr
> [  5]   0.00-10.00  sec  2.74 GBytes  2.36 Gbits/sec    0             sender
> [  5]   0.00-10.04  sec  2.74 GBytes  2.34 Gbits/sec                  receiver
>
> Co-developed-by: Lai Peter Jun Ann <[email-masked]>
> Signed-off-by: Lai Peter Jun Ann <[email-masked]>
> Signed-off-by: Song Yoong Siang <[email-masked]>
> Acked-by: John Fastabend <[email-masked]>
> ---
> V1: https://patchwork.kernel.org/project/netdevbpf/patch/[email-masked]/
> V2: https://patchwork.kernel.org/project/netdevbpf/cover/[email-masked]/
> V3: https://patchwork.kernel.org/project/netdevbpf/cover/[email-masked]/
>
> changelog:
> V1 -> V2
> - In struct igc_tx_timestamp_request, keep a pointer to igc_tx_buffer,
>   instead of pointing xsk_pending_ts (Vinicius).
> - In struct igc_tx_timestamp_request, introduce buffer_type to indicate
>   whether skb or igc_tx_buffer pointer should be use (Vinicius).
> - In struct igc_metadata_request, remove igc_adapter pointer (Vinicius).
> - When request tx hwts, copy the value of cmd_type, instead of using
>   pointer (Vinicius).
> - For boolean variable, use true and false, instead of 1 and 0 (Vinicius).
> - In igc_xsk_request_timestamp(), make an early return if none of the 4 ts
>   registers is available (Vinicius).
> - Create helper functions to clear tx buffer and skb for tstamp (John).
> - Perform throughput test with mix traffic (Vinicius & John).
> V2 -> V3
> - Improve tstamp reg searching loop for better readability (John).
> - In igc_ptp_free_tx_buffer(), add comment to inform user that
>   tstamp->xsk_tx_buffer and tstamp->skb are in union (John).
> V3 -> V4
> - Add protection with xp_tx_metadata_enabled (Kurt & Maciej).
> ---
> ---
>  drivers/net/ethernet/intel/igc/igc.h      |  71 ++++++++------
>  drivers/net/ethernet/intel/igc/igc_main.c | 113 ++++++++++++++++++++--
>  drivers/net/ethernet/intel/igc/igc_ptp.c  |  51 ++++++++--
>  3 files changed, 195 insertions(+), 40 deletions(-)
>

[...]

> diff --git a/drivers/net/ethernet/intel/igc/igc_ptp.c b/drivers/net/ethernet/intel/igc/igc_ptp.c
> index 885faaa7b9de..1bb026232efc 100644
> --- a/drivers/net/ethernet/intel/igc/igc_ptp.c
> +++ b/drivers/net/ethernet/intel/igc/igc_ptp.c
> @@ -11,6 +11,7 @@
>  #include <linux/ktime.h>
>  #include <linux/delay.h>
>  #include <linux/iopoll.h>
> +#include <net/xdp_sock_drv.h>
>  
>  #define INCVALUE_MASK		0x7fffffff
>  #define ISGN			0x80000000
> @@ -545,6 +546,30 @@ static void igc_ptp_enable_rx_timestamp(struct igc_adapter *adapter)
>  	wr32(IGC_TSYNCRXCTL, val);
>  }
>  
> +static void igc_ptp_free_tx_buffer(struct igc_adapter *adapter,
> +				   struct igc_tx_timestamp_request *tstamp)
> +{
> +	if (tstamp->buffer_type == IGC_TX_BUFFER_TYPE_XSK) {
> +		/* Release the transmit completion */
> +		tstamp->xsk_tx_buffer->xsk_pending_ts = false;
> +
> +		/* Note: tstamp->skb and tstamp->xsk_tx_buffer are in union.
> +		 * By setting tstamp->xsk_tx_buffer to NULL, tstamp->skb will
> +		 * become NULL as well.
> +		 */
> +		tstamp->xsk_tx_buffer = NULL;
> +		tstamp->buffer_type = 0;
> +
> +		/* Trigger txrx interrupt for transmit completion */
> +		igc_xsk_wakeup(adapter->netdev, tstamp->xsk_queue_index, 0);
> +
> +		return;
> +	}
> +
> +	dev_kfree_skb_any(tstamp->skb);
> +	tstamp->skb = NULL;
> +}
> +
>  static void igc_ptp_clear_tx_tstamp(struct igc_adapter *adapter)
>  {
>  	unsigned long flags;
> @@ -555,8 +580,8 @@ static void igc_ptp_clear_tx_tstamp(struct igc_adapter *adapter)
>  	for (i = 0; i < IGC_MAX_TX_TSTAMP_REGS; i++) {
>  		struct igc_tx_timestamp_request *tstamp = &adapter->tx_tstamp[i];
>  
> -		dev_kfree_skb_any(tstamp->skb);
> -		tstamp->skb = NULL;
> +		if (tstamp->skb)
> +			igc_ptp_free_tx_buffer(adapter, tstamp);
>  	}
>  

More a question: you are potentially triggering an interrupt from
igc_ptp_clear_tx_tstamp() (igc_xsk_wakeup()) which can be called from
igc_down(). So, how does it work when there's a pending timestamp and
you remove the igc module? (example of a situation that it might be
problematic).

Cheers,
-- 
Vinicius