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

Jesper Dangaard Brouer
2023-04-18T13:32:13Z

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 <brouer@redhat.com>
---
.../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();
+ 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;