Subject: Re: [PATCH bpf-next V2 5/5] selftests/bpf: xdp_hw_metadata track more timestamps

From: Stanislav Fomichev <[email-masked]>
Date: Tue, 18 Apr 2023 09:36:55 -0700

On 04/18, Jesper Dangaard Brouer wrote:
> To correlate the hardware RX timestamp with something, add tracking of
> two software timestamps both clock source CLOCK_TAI (see description in
> man clock_gettime(2)).
> 
> XDP metadata is extended with xdp_timestamp for capturing when XDP
> received the packet. Populated with BPF helper bpf_ktime_get_tai_ns(). I
> could not find a BPF helper for getting CLOCK_REALTIME, which would have
> been preferred. In userspace when AF_XDP sees the packet another
> software timestamp is recorded via clock_gettime() also clock source
> CLOCK_TAI.
> 
> Example output shortly after loading igc driver:
> 
>   poll: 1 (0) skip=1 fail=0 redir=2
>   xsk_ring_cons__peek: 1
>   0x12557a8: rx_desc[1]->addr=100000000009000 addr=9100 comp_addr=9000
>   rx_hash: 0x82A96531 with RSS type:0x1
>   rx_timestamp:  1681740540304898909 (sec:1681740540.3049)
>   XDP RX-time:   1681740577304958316 (sec:1681740577.3050) delta sec:37.0001 (37000059.407 usec)
>   AF_XDP time:   1681740577305051315 (sec:1681740577.3051) delta sec:0.0001 (92.999 usec)
>   0x12557a8: complete idx=9 addr=9000
> 
> The first observation is that the 37 sec difference between RX HW vs XDP
> timestamps, which indicate hardware is likely clock source
> CLOCK_REALTIME, because (as of this writing) CLOCK_TAI is initialised
> with a 37 sec offset.
> 
> The 93 usec (microsec) difference between XDP vs AF_XDP userspace is the
> userspace wakeup time. On this hardware it was caused by CPU idle sleep
> states, which can be reduced by tuning /dev/cpu_dma_latency.
> 
> View current requested/allowed latency bound via:
>   hexdump --format '"%d\n"' /dev/cpu_dma_latency
> 
> More explanation of the output and how this can be used to identify
> clock drift for the HW clock can be seen here[1]:
> 
> [1] https://github.com/xdp-project/xdp-project/blob/master/areas/hints/xdp_hints_kfuncs02_driver_igc.org
> 
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>

Acked-by: Stanislav Fomichev <[email-masked]>

> ---
>  .../testing/selftests/bpf/progs/xdp_hw_metadata.c  |    4 +-
>  tools/testing/selftests/bpf/xdp_hw_metadata.c      |   47 ++++++++++++++++++--
>  tools/testing/selftests/bpf/xdp_metadata.h         |    1 
>  3 files changed, 46 insertions(+), 6 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> index e1c787815e44..b2dfd7066c6e 100644
> --- a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> +++ b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> @@ -77,7 +77,9 @@ int rx(struct xdp_md *ctx)
>  	}
>  
>  	err = bpf_xdp_metadata_rx_timestamp(ctx, &meta->rx_timestamp);
> -	if (err)

[..]

> +	if (!err)
> +		meta->xdp_timestamp = bpf_ktime_get_tai_ns();

nit: why not set it unconditionally?

> +	else
>  		meta->rx_timestamp = 0; /* Used by AF_XDP as not avail signal */
>  
>  	err = bpf_xdp_metadata_rx_hash(ctx, &meta->rx_hash, &meta->rx_hash_type);
> diff --git a/tools/testing/selftests/bpf/xdp_hw_metadata.c b/tools/testing/selftests/bpf/xdp_hw_metadata.c
> index 987cf0db5ebc..613321eb84c1 100644
> --- a/tools/testing/selftests/bpf/xdp_hw_metadata.c
> +++ b/tools/testing/selftests/bpf/xdp_hw_metadata.c
> @@ -27,6 +27,7 @@
>  #include <sys/mman.h>
>  #include <net/if.h>
>  #include <poll.h>
> +#include <time.h>
>  
>  #include "xdp_metadata.h"
>  
> @@ -134,18 +135,52 @@ static void refill_rx(struct xsk *xsk, __u64 addr)
>  	}
>  }
>  
> -static void verify_xdp_metadata(void *data)
> +#define NANOSEC_PER_SEC 1000000000 /* 10^9 */
> +static __u64 gettime(clockid_t clock_id)
> +{
> +	struct timespec t;
> +	int res;
> +
> +	/* See man clock_gettime(2) for type of clock_id's */
> +	res = clock_gettime(clock_id, &t);
> +
> +	if (res < 0)
> +		error(res, errno, "Error with clock_gettime()");
> +
> +	return (__u64) t.tv_sec * NANOSEC_PER_SEC + t.tv_nsec;
> +}
> +
> +static void verify_xdp_metadata(void *data, clockid_t clock_id)
>  {
>  	struct xdp_meta *meta;
>  
>  	meta = data - sizeof(*meta);
>  
> -	printf("rx_timestamp: %llu\n", meta->rx_timestamp);
>  	if (meta->rx_hash_err < 0)
>  		printf("No rx_hash err=%d\n", meta->rx_hash_err);
>  	else
>  		printf("rx_hash: 0x%X with RSS type:0x%X\n",
>  		       meta->rx_hash, meta->rx_hash_type);
> +
> +	printf("rx_timestamp:  %llu (sec:%0.4f)\n", meta->rx_timestamp,
> +	       (double)meta->rx_timestamp / NANOSEC_PER_SEC);
> +	if (meta->rx_timestamp) {
> +		__u64 usr_clock = gettime(clock_id);
> +		__u64 xdp_clock = meta->xdp_timestamp;
> +		__s64 delta_X = xdp_clock - meta->rx_timestamp;
> +		__s64 delta_X2U = usr_clock - xdp_clock;
> +
> +		printf("XDP RX-time:   %llu (sec:%0.4f) delta sec:%0.4f (%0.3f usec)\n",
> +		       xdp_clock, (double)xdp_clock / NANOSEC_PER_SEC,
> +		       (double)delta_X / NANOSEC_PER_SEC,
> +		       (double)delta_X / 1000);
> +
> +		printf("AF_XDP time:   %llu (sec:%0.4f) delta sec:%0.4f (%0.3f usec)\n",
> +		       usr_clock, (double)usr_clock / NANOSEC_PER_SEC,
> +		       (double)delta_X2U / NANOSEC_PER_SEC,
> +		       (double)delta_X2U / 1000);
> +	}
> +
>  }
>  
>  static void verify_skb_metadata(int fd)
> @@ -193,7 +228,7 @@ static void verify_skb_metadata(int fd)
>  	printf("skb hwtstamp is not found!\n");
>  }
>  
> -static int verify_metadata(struct xsk *rx_xsk, int rxq, int server_fd)
> +static int verify_metadata(struct xsk *rx_xsk, int rxq, int server_fd, clockid_t clock_id)
>  {
>  	const struct xdp_desc *rx_desc;
>  	struct pollfd fds[rxq + 1];
> @@ -243,7 +278,8 @@ static int verify_metadata(struct xsk *rx_xsk, int rxq, int server_fd)
>  			addr = xsk_umem__add_offset_to_addr(rx_desc->addr);
>  			printf("%p: rx_desc[%u]->addr=%llx addr=%llx comp_addr=%llx\n",
>  			       xsk, idx, rx_desc->addr, addr, comp_addr);
> -			verify_xdp_metadata(xsk_umem__get_data(xsk->umem_area, addr));
> +			verify_xdp_metadata(xsk_umem__get_data(xsk->umem_area, addr),
> +					    clock_id);
>  			xsk_ring_cons__release(&xsk->rx, 1);
>  			refill_rx(xsk, comp_addr);
>  		}
> @@ -370,6 +406,7 @@ static void timestamping_enable(int fd, int val)
>  
>  int main(int argc, char *argv[])
>  {
> +	clockid_t clock_id = CLOCK_TAI;
>  	int server_fd = -1;
>  	int ret;
>  	int i;
> @@ -443,7 +480,7 @@ int main(int argc, char *argv[])
>  		error(1, -ret, "bpf_xdp_attach");
>  
>  	signal(SIGINT, handle_signal);
> -	ret = verify_metadata(rx_xsk, rxq, server_fd);
> +	ret = verify_metadata(rx_xsk, rxq, server_fd, clock_id);
>  	close(server_fd);
>  	cleanup();
>  	if (ret)
> diff --git a/tools/testing/selftests/bpf/xdp_metadata.h b/tools/testing/selftests/bpf/xdp_metadata.h
> index 0c4624dc6f2f..938a729bd307 100644
> --- a/tools/testing/selftests/bpf/xdp_metadata.h
> +++ b/tools/testing/selftests/bpf/xdp_metadata.h
> @@ -11,6 +11,7 @@
>  
>  struct xdp_meta {
>  	__u64 rx_timestamp;
> +	__u64 xdp_timestamp;
>  	__u32 rx_hash;
>  	union {
>  		__u32 rx_hash_type;
> 
>