On Thu, 3 Sept 2026 at 06:02, Andrii Nakryiko <[email protected]> wrote: > > 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 > >
Hi Andrii, I did use AI to find this. I often go through recent commits and then ask AI to check whether the related selftests have been updated and that's how I found this.
