sched_ext: create_dsq: Return -EEXIST on duplicate request

create_dsq and therefore the scx_bpf_create_dsq kfunc currently silently
ignore duplicate entries. As a sched_ext scheduler is creating each DSQ
for a different purpose this is surprising behaviour.

Replace rhashtable_insert_fast which ignores duplicates with
rhashtable_lookup_insert_fast that reports duplicates (though doesn't
return their value). The rest of the code is structured correctly and
this now returns -EEXIST.

Tested by adding an extra scx_bpf_create_dsq to scx_simple. Previously
this was ignored, now init fails with a -17 code. Also ran scx_lavd
which continued to work well.

Signed-off-by: Jake Hillion <[email protected]>
Acked-by: Andrea Righi <[email protected]>
Fixes: f0e1a06 ("sched_ext: Implement BPF extensible scheduler class")
Cc: [email protected] # v6.12+
Signed-off-by: Tejun Heo <[email protected]>

Author: JakeHillion

kernel/sched/ext.c

@@ -4171,8 +4171,8 @@ static struct scx_dispatch_q *create_dsq(u64 dsq_id, int node)
 
 	init_dsq(dsq, dsq_id);
 
-	ret = rhashtable_insert_fast(&dsq_hash, &dsq->hash_node,
-				     dsq_hash_params);
+	ret = rhashtable_lookup_insert_fast(&dsq_hash, &dsq->hash_node,
+					    dsq_hash_params);
 	if (ret) {
 		kfree(dsq);
 		return ERR_PTR(ret);