On Tue, Sep 1, 2026 at 7:20 AM Suchit Karunakaran
<[email protected]> wrote:
>
> libbpf_num_possible_cpus() returns the number of possible CPUs, which is
> appropriate for sizing packed per-CPU map value buffers. It is not the
> upper bound for logical CPU IDs.
>
> For a possible CPU mask such as 0,2-3, the test loops over CPU IDs 0
> through 2. This incorrectly visits CPU 1 and misses CPU 3. It also
> initializes packed per-CPU slots using their slot indexes rather than
> the corresponding logical CPU IDs.
>
> Parse the possible CPU mask and keep the packed slot count separate
> from the logical CPU ID range. Populate each dense per-CPU slot with
> its logical CPU ID, calculate the corresponding expected sum, and make
> the BPF program iterate over the full CPU ID range.
>
Is this the issue you ran into in practice or it's just another of
those found by AI reading code?
> Fixes: 7aa424e02a04bba5ecc84afe9b58b16e9e0b34f8 ("selftests/bpf: Fix some
> bugs in map_lookup_percpu_elem testcase")
>
> Signed-off-by: Suchit Karunakaran <[email protected]>
> ---
> .../bpf/prog_tests/map_lookup_percpu_elem.c | 34 ++++++++++++++-----
> .../bpf/progs/test_map_lookup_percpu_elem.c | 8 ++---
> 2 files changed, 30 insertions(+), 12 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/map_lookup_percpu_elem.c
> b/tools/testing/selftests/bpf/prog_tests/map_lookup_percpu_elem.c
> index bfb1bf3fd427..849f33259c00 100644
> --- a/tools/testing/selftests/bpf/prog_tests/map_lookup_percpu_elem.c
> +++ b/tools/testing/selftests/bpf/prog_tests/map_lookup_percpu_elem.c
> @@ -2,29 +2,45 @@
> /* Copyright (c) 2022 Bytedance */
>
> #include <test_progs.h>
> +#include "bpf/libbpf_internal.h"
> #include "test_map_lookup_percpu_elem.skel.h"
>
> void test_map_lookup_percpu_elem(void)
> {
> struct test_map_lookup_percpu_elem *skel;
> - __u64 key = 0, sum;
> - int ret, i, nr_cpus = libbpf_num_possible_cpus();
> + bool *possible = NULL;
> + __u64 key = 0, sum = 0;
> + int cpu, nr_cpu_ids, nr_cpus, ret, slot = 0;
> __u64 *buf;
>
> - buf = malloc(nr_cpus*sizeof(__u64));
> - if (!ASSERT_OK_PTR(buf, "malloc"))
> + ret = parse_cpu_mask_file("/sys/devices/system/cpu/possible",
> &possible,
> + &nr_cpu_ids);
> + if (!ASSERT_OK(ret, "parse possible CPU mask"))
> return;
>
> - for (i = 0; i < nr_cpus; i++)
> - buf[i] = i;
> - sum = (nr_cpus - 1) * nr_cpus / 2;
> + nr_cpus = libbpf_num_possible_cpus();
> + if (!ASSERT_GT(nr_cpus, 0, "libbpf_num_possible_cpus"))
> + goto free_mask;
> +
> + buf = malloc(nr_cpus * sizeof(*buf));
> + if (!ASSERT_OK_PTR(buf, "malloc"))
> + goto free_mask;
> +
> + for (cpu = 0; cpu < nr_cpu_ids; cpu++) {
> + if (!possible[cpu])
> + continue;
> + buf[slot++] = cpu;
> + sum += cpu;
> + }
> + if (!ASSERT_EQ(slot, nr_cpus, "possible CPU mask weight"))
> + goto exit;
>
> skel = test_map_lookup_percpu_elem__open();
> if (!ASSERT_OK_PTR(skel, "test_map_lookup_percpu_elem__open"))
> goto exit;
>
> skel->rodata->my_pid = getpid();
> - skel->rodata->nr_cpus = nr_cpus;
> + skel->rodata->nr_cpu_ids = nr_cpu_ids;
>
> ret = test_map_lookup_percpu_elem__load(skel);
> if (!ASSERT_OK(ret, "test_map_lookup_percpu_elem__load"))
> @@ -55,4 +71,6 @@ void test_map_lookup_percpu_elem(void)
> test_map_lookup_percpu_elem__destroy(skel);
> exit:
> free(buf);
> +free_mask:
> + free(possible);
> }
> diff --git a/tools/testing/selftests/bpf/progs/test_map_lookup_percpu_elem.c
> b/tools/testing/selftests/bpf/progs/test_map_lookup_percpu_elem.c
> index ca827b1092da..d8da0696b97c 100644
> --- a/tools/testing/selftests/bpf/progs/test_map_lookup_percpu_elem.c
> +++ b/tools/testing/selftests/bpf/progs/test_map_lookup_percpu_elem.c
> @@ -7,7 +7,7 @@
> __u64 percpu_array_elem_sum = 0;
> __u64 percpu_hash_elem_sum = 0;
> __u64 percpu_lru_hash_elem_sum = 0;
> -const volatile int nr_cpus;
> +const volatile int nr_cpu_ids;
> const volatile int my_pid;
>
> struct {
> @@ -57,17 +57,17 @@ int sysenter_getuid(const void *ctx)
>
> map_ctx.map = &percpu_array_map;
> map_ctx.sum = 0;
> - bpf_loop(nr_cpus, read_percpu_elem_callback, &map_ctx, 0);
> + bpf_loop(nr_cpu_ids, read_percpu_elem_callback, &map_ctx, 0);
> percpu_array_elem_sum = map_ctx.sum;
>
> map_ctx.map = &percpu_hash_map;
> map_ctx.sum = 0;
> - bpf_loop(nr_cpus, read_percpu_elem_callback, &map_ctx, 0);
> + bpf_loop(nr_cpu_ids, read_percpu_elem_callback, &map_ctx, 0);
> percpu_hash_elem_sum = map_ctx.sum;
>
> map_ctx.map = &percpu_lru_hash_map;
> map_ctx.sum = 0;
> - bpf_loop(nr_cpus, read_percpu_elem_callback, &map_ctx, 0);
> + bpf_loop(nr_cpu_ids, read_percpu_elem_callback, &map_ctx, 0);
> percpu_lru_hash_elem_sum = map_ctx.sum;
>
> return 0;
> --
> 2.55.0
>
