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

[Zhao Mengmeng]

From: Zhao Mengmeng <zhaomengmeng@xxxxxxxxxx>

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@xxxxxxxxxx>
---
 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