[PATCH] tools/sched_ext: scx_pair: fix pair_ctx indexing for CPU pairs

Zhao Mengmeng
2026-03-26T03:06:08Z

From: Zhao Mengmeng <zhaomengmeng@kylinos.cn>

scx_pair sizes pair_ctx to nr_cpu_ids / 2, so valid pair_ctx keys are
dense pair indexes in the range [0, nr_cpu_ids / 2).

However, the userspace setup code stores pair_id as the first CPU number
in each pair. On an 8-CPU system with "-S 1", that produces pair IDs
0, 2, 4 and 6 for pairs [0,1], [2,3], [4,5] and [6,7]. CPUs in the
latter half then look up pair_ctx with out-of-range keys and the BPF
scheduler aborts with:

EXIT: scx_bpf_error (scx_pair.bpf.c:328: failed to lookup pairc and
in_pair_mask for cpu[5])

Assign pair_id using a dense pair counter instead so that each CPU pair
maps to a valid pair_ctx entry. Besides, reject odd CPU configuration, as
scx_pair requires all CPUs to be paired.

Fixes: f0262b102c7c ("tools/sched_ext: add scx_pair scheduler")
Signed-off-by: Zhao Mengmeng <zhaomengmeng@kylinos.cn>
---
tools/sched_ext/scx_pair.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/tools/sched_ext/scx_pair.c b/tools/sched_ext/scx_pair.c
index 61fd86b44c40..41b136d43a55 100644
--- a/tools/sched_ext/scx_pair.c
+++ b/tools/sched_ext/scx_pair.c
@@ -48,6 +48,7 @@ int main(int argc, char **argv)
struct bpf_link *link;
__u64 seq = 0, ecode;
__s32 stride, i, opt, outer_fd;
+ __u32 pair_id = 0;

libbpf_set_print(libbpf_print_fn);
signal(SIGINT, sigint_handler);
@@ -82,6 +83,14 @@ int main(int argc, char **argv)
scx_pair__destroy(skel);
return -1;
}
+
+ if (skel->rodata->nr_cpu_ids & 1) {
+ fprintf(stderr, "scx_pair requires an even CPU count, got %u\n",
+ skel->rodata->nr_cpu_ids);
+ scx_pair__destroy(skel);
+ return -1;
+ }
+
bpf_map__set_max_entries(skel->maps.pair_ctx, skel->rodata->nr_cpu_ids / 2);

/* Resize arrays so their element count is equal to cpu count. */
@@ -109,10 +118,11 @@ int main(int argc, char **argv)

skel->rodata_pair_cpu->pair_cpu[i] = j;
skel->rodata_pair_cpu->pair_cpu[j] = i;
- skel->rodata_pair_id->pair_id[i] = i;
- skel->rodata_pair_id->pair_id[j] = i;
+ skel->rodata_pair_id->pair_id[i] = pair_id;
+ skel->rodata_pair_id->pair_id[j] = pair_id;
skel->rodata_in_pair_idx->in_pair_idx[i] = 0;
skel->rodata_in_pair_idx->in_pair_idx[j] = 1;
+ pair_id++;

printf("[%d, %d] ", i, j);
}
--
2.43.0