[PATCH 2/2] sched_ext: Specialize the scheduler hashtable compare

[Usama Arif <usama.arif@linux.dev>]

scx_sched_hash_params does not provide an object comparison function,
so rhashtable falls back to rhashtable_compare(). Although the
sub-cgroup ID key is one naturally aligned u64, the generic comparison
reads the key offset and length from the table parameters at runtime and
emits an out-of-line memcmp() for each element walked.

Supply an obj_cmpfn so lookups specialize to a single equality test
against ops.sub_cgroup_id. ext.c and sub.c are included in the same
build_policy.c translation unit, so the compiler can fold the const
callback into scx_find_sub_sched() even though its source is in sub.c.

The result is only tested against zero, so the ordering provided by
memcmp() is not observable. No functional change intended.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Usama Arif <usama.arif@linux.dev>
---
 kernel/sched/ext/ext.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 288d6b80bbca0..87fa09c71eb0f 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -41,10 +41,21 @@ struct scx_sched __rcu *scx_root;
 LIST_HEAD(scx_sched_all);
 
 #ifdef CONFIG_EXT_SUB_SCHED
+static __always_inline int scx_sched_cmpfn(struct rhashtable_compare_arg *arg,
+					   const void *ptr)
+{
+	const struct scx_sched *sch = ptr;
+
+	BUILD_BUG_ON(sizeof_field(struct scx_sched, ops.sub_cgroup_id) != sizeof(u64));
+
+	return sch->ops.sub_cgroup_id != *(const u64 *)arg->key;
+}
+
 const struct rhashtable_params scx_sched_hash_params = {
 	.key_len		= sizeof_field(struct scx_sched, ops.sub_cgroup_id),
 	.key_offset		= offsetof(struct scx_sched, ops.sub_cgroup_id),
 	.head_offset		= offsetof(struct scx_sched, hash_node),
+	.obj_cmpfn		= scx_sched_cmpfn,
 	.insecure_elasticity	= true,	/* inserted under scx_sched_lock */
 };
 
-- 
2.53.0-Meta