Subject: Re: [PATCH bpf-next v3 5/6] selftests: bpf: add xdp_perf test

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 10 Dec 2019 12:05:09 +0100

On Mon,  9 Dec 2019 14:55:21 +0100
Björn Töpel <[email-masked]> wrote:

> From: Björn Töpel <[email-masked]>
> 
> The xdp_perf is a dummy XDP test, only used to measure the the cost of
> jumping into a XDP program.

I really like this idea of performance measuring XDP-core in isolation.
This is the ultimate zoom-in micro-benchmarking.  I see a use-case for
this, where I will measure the XDP-core first, and then run same XDP
prog (e.g. XDP_DROP) on a NIC driver, then I can deduct/isolate the
driver-code and hardware overhead.  We/I can also use it to optimize
e.g. REDIRECT code-core (although redir might not actually work).

IMHO it would be valuable to have bpf_prog_load() also measure the
perf-HW counters for 'cycles' and 'instructions', as in your case the
performance optimization was to improve the instructions-per-cycle
(which you showed via perf stat in cover letter).

If you send a V4 please describe how to use this prog to measure the
cost, as you describe in cover letter.

from selftests/bpf run:
 # test_progs -v -t xdp_perf

(This is a nitpick, so only do this if something request a V4)

> Signed-off-by: Björn Töpel <[email-masked]>
> ---
>  .../selftests/bpf/prog_tests/xdp_perf.c       | 25 +++++++++++++++++++
>  1 file changed, 25 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/xdp_perf.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_perf.c b/tools/testing/selftests/bpf/prog_tests/xdp_perf.c
> new file mode 100644
> index 000000000000..7185bee16fe4
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_perf.c
> @@ -0,0 +1,25 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <test_progs.h>
> +
> +void test_xdp_perf(void)
> +{
> +	const char *file = "./xdp_dummy.o";
> +	__u32 duration, retval, size;
> +	struct bpf_object *obj;
> +	char in[128], out[128];
> +	int err, prog_fd;
> +
> +	err = bpf_prog_load(file, BPF_PROG_TYPE_XDP, &obj, &prog_fd);
> +	if (CHECK_FAIL(err))
> +		return;
> +
> +	err = bpf_prog_test_run(prog_fd, 1000000, &in[0], 128,
> +				out, &size, &retval, &duration);
> +
> +	CHECK(err || retval != XDP_PASS || size != 128,
> +	      "xdp-perf",
> +	      "err %d errno %d retval %d size %d\n",
> +	      err, errno, retval, size);
> +
> +	bpf_object__close(obj);
> +}

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer