tools/sched_ext: scx_pair: fix pair_ctx indexing for CPU pairs

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>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/tools/sched_ext/scx_pair.c b/tools/sched_ext/scx_pair.c
index 61fd86b44c40fa4ade5b9e9f3bf37fec2cfa0f19..41b136d43a55540a99eee67235ab65b7adf46667 100644 (file)
--- 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 @@ restart:
                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 @@ restart:
 
                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);
        }