Re: [PATCH bpf-next v3 7/7] selftests/bpf: BPF task work scheduling tests

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

On 9/8/25 08:43, Eduard Zingerman wrote:
> On Fri, 2025-09-05 at 17:45 +0100, Mykyta Yatsenko wrote:
>> From: Mykyta Yatsenko <yatsenko@meta.com>
>>
>> Introducing selftests that check BPF task work scheduling mechanism.
>> Validate that verifier does not accepts incorrect calls to
>> bpf_task_work_schedule kfunc.
>>
>> Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
>> ---
> The test cases in this patch check functional correctness, but there
> is no attempt to do some stress testing of the state machine.
> E.g. how hard/feasible would it be to construct a test that attempts
> to exercise both branches of the (state == BPF_TW_SCHEDULED) in the
> bpf_task_work_cancel_and_free()?
Good point, I have stress test, but did not include it in the patches, 
as it takes longer to run.
I had to add logs in the kernel code to make sure cancellation/freeing 
branches are hit.
https://github.com/kernel-patches/bpf/commit/86408b074ab0a2d290977846c3e99a07443ac604
>
>>   .../selftests/bpf/prog_tests/test_task_work.c | 149 ++++++++++++++++++
>>   tools/testing/selftests/bpf/progs/task_work.c | 108 +++++++++++++
>>   .../selftests/bpf/progs/task_work_fail.c      |  98 ++++++++++++
>>   3 files changed, 355 insertions(+)
>>   create mode 100644 tools/testing/selftests/bpf/prog_tests/test_task_work.c
>>   create mode 100644 tools/testing/selftests/bpf/progs/task_work.c
>>   create mode 100644 tools/testing/selftests/bpf/progs/task_work_fail.c
>>
>> diff --git a/tools/testing/selftests/bpf/prog_tests/test_task_work.c b/tools/testing/selftests/bpf/prog_tests/test_task_work.c
>> new file mode 100644
>> index 000000000000..9c3c7a46a827
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/prog_tests/test_task_work.c
>> @@ -0,0 +1,149 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
>> +#include <test_progs.h>
>> +#include <string.h>
>> +#include <stdio.h>
>> +#include "task_work.skel.h"
>> +#include "task_work_fail.skel.h"
>> +#include <linux/bpf.h>
>> +#include <linux/perf_event.h>
>> +#include <sys/syscall.h>
>> +#include <time.h>
>> +
>> +static int perf_event_open(__u32 type, __u64 config, int pid)
>> +{
>> +	struct perf_event_attr attr = {
>> +		.type = type,
>> +		.config = config,
>> +		.size = sizeof(struct perf_event_attr),
>> +		.sample_period = 100000,
>> +	};
>> +
>> +	return syscall(__NR_perf_event_open, &attr, pid, -1, -1, 0);
>> +}
>> +
>> +struct elem {
>> +	char data[128];
>> +	struct bpf_task_work tw;
>> +};
>> +
>> +static int verify_map(struct bpf_map *map, const char *expected_data)
>> +{
>> +	int err;
>> +	struct elem value;
>> +	int processed_values = 0;
>> +	int k, sz;
>> +
>> +	sz = bpf_map__max_entries(map);
>> +	for (k = 0; k < sz; ++k) {
>> +		err = bpf_map__lookup_elem(map, &k, sizeof(int), &value, sizeof(struct elem), 0);
>> +		if (err)
>> +			continue;
>> +		if (!ASSERT_EQ(strcmp(expected_data, value.data), 0, "map data")) {
>> +			fprintf(stderr, "expected '%s', found '%s' in %s map", expected_data,
>> +				value.data, bpf_map__name(map));
>> +			return 2;
>> +		}
>> +		processed_values++;
>> +	}
>> +
>> +	return processed_values == 0;
> Nit: check for exact number of expected values here?
>
>> +}
>> +
>> +static void task_work_run(const char *prog_name, const char *map_name)
>> +{
>> +	struct task_work *skel;
>> +	struct bpf_program *prog;
>> +	struct bpf_map *map;
>> +	struct bpf_link *link;
>> +	int err, pe_fd = 0, pid, status, pipefd[2];
>> +	char user_string[] = "hello world";
>> +
>> +	if (!ASSERT_NEQ(pipe(pipefd), -1, "pipe"))
>> +		return;
>> +
>> +	pid = fork();
> Nit: check for negative return value?
>
>> +	if (pid == 0) {
>> +		__u64 num = 1;
>> +		int i;
>> +		char buf;
>> +
>> +		close(pipefd[1]);
>> +		read(pipefd[0], &buf, sizeof(buf));
>> +		close(pipefd[0]);
>> +
>> +		for (i = 0; i < 10000; ++i)
>> +			num *= time(0) % 7;
>> +		(void)num;
>> +		exit(0);
>> +	}
>> +	skel = task_work__open();
>> +	if (!ASSERT_OK_PTR(skel, "task_work__open"))
>> +		return;
>> +
>> +	bpf_object__for_each_program(prog, skel->obj) {
>> +		bpf_program__set_autoload(prog, false);
>> +	}
>> +
>> +	prog = bpf_object__find_program_by_name(skel->obj, prog_name);
>> +	if (!ASSERT_OK_PTR(prog, "prog_name"))
>> +		goto cleanup;
>> +	bpf_program__set_autoload(prog, true);
>> +	bpf_program__set_type(prog, BPF_PROG_TYPE_PERF_EVENT);
> Nit: this is not really necessary, the programs are already defined as
>       SEC("perf_event").
>
>> +	skel->bss->user_ptr = (char *)user_string;
>> +
>> +	err = task_work__load(skel);
>> +	if (!ASSERT_OK(err, "skel_load"))
>> +		goto cleanup;
> [...]