[PATCH 1/2] sched_ext: Specialize the TID hashtable compare

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

scx_tid_hash_params does not provide an object comparison function, so
rhashtable falls back to rhashtable_compare(). Although the 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.

scx_bpf_tid_to_task() can sit on hot scheduling paths. Supply an
obj_cmpfn so the const parameters passed into the inlined rhashtable
lookup specialize the comparison to a single equality test against
scx->tid.

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 646affba4e3c4..288d6b80bbca0 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -52,10 +52,21 @@ struct rhashtable scx_sched_hash;
 #endif
 
 /* see SCX_OPS_TID_TO_TASK */
+static __always_inline int scx_tid_cmpfn(struct rhashtable_compare_arg *arg,
+					 const void *ptr)
+{
+	const struct sched_ext_entity *scx = ptr;
+
+	BUILD_BUG_ON(sizeof_field(struct sched_ext_entity, tid) != sizeof(u64));
+
+	return scx->tid != *(const u64 *)arg->key;
+}
+
 static const struct rhashtable_params scx_tid_hash_params = {
 	.key_len		= sizeof_field(struct sched_ext_entity, tid),
 	.key_offset		= offsetof(struct sched_ext_entity, tid),
 	.head_offset		= offsetof(struct sched_ext_entity, tid_hash_node),
+	.obj_cmpfn		= scx_tid_cmpfn,
 	.insecure_elasticity	= true,	/* inserted/removed under scx_tasks_lock */
 };
 static struct rhashtable scx_tid_hash;
-- 
2.53.0-Meta