[PATCH] sched_ext: Specialize the DSQ hashtable compare

[Usama Arif]

rhashtable is a generic container: it does not know what a key is, so it
carries the key's shape as data. dsq_hash_params declares key_len,
key_offset and head_offset and nothing else, so lookups fall back to
rhashtable_compare(), which reads both the offset and the length back out
of ht->p at runtime:

	memcmp(ptr + ht->p.key_offset, arg->key, ht->p.key_len)

The key is one naturally aligned u64, but neither the offset nor the
length is a compile-time constant there, so the compiler cannot narrow
the call and emits an out-of-line memcmp() for every element walked -
two loads, a call and a length dispatch to compare eight bytes. The
interpretation costs more than the comparison it is interpreting.

find_user_dsq() sits on the __schedule() path, and scx_layered calls
scx_bpf_dsq_nr_queued() once per layer per dispatch decision. On Meta's
fleet the lookup has a significant cost and shows up in fleet wide
profile.

Let's supply an obj_cmpfn. dsq_hash_params is a const object passed by
value into the __always_inline __rhashtable_lookup(), so
params.obj_cmpfn is a compile-time constant, the ternary that selects it
folds away and the callback inlines: the compare becomes a single cmp
against dsq->id. Every consumer of the result only tests it against
zero, so the ordering memcmp() also carries is never observed. No
functional change intended.

An in-kernel A/B over the DSQ ids scx_layered creates, both parameter
sets compiled into the same kernel, makes the lookup 2.9x faster.

Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
---
 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 9f40f366a1c13..646affba4e3c4 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -189,10 +189,21 @@ static DEFINE_PER_CPU(struct scx_tid_alloc, scx_tid_alloc);
  */
 static DEFINE_PER_CPU(struct task_struct *, direct_dispatch_task);
 
+static __always_inline int dsq_cmpfn(struct rhashtable_compare_arg *arg,
+				     const void *ptr)
+{
+	const struct scx_dispatch_q *dsq = ptr;
+
+	BUILD_BUG_ON(sizeof_field(struct scx_dispatch_q, id) != sizeof(u64));
+
+	return dsq->id != *(const u64 *)arg->key;
+}
+
 static const struct rhashtable_params dsq_hash_params = {
 	.key_len		= sizeof_field(struct scx_dispatch_q, id),
 	.key_offset		= offsetof(struct scx_dispatch_q, id),
 	.head_offset		= offsetof(struct scx_dispatch_q, hash_node),
+	.obj_cmpfn		= dsq_cmpfn,
 };
 
 static LLIST_HEAD(dsqs_to_free);

base-commit: d9ecc8c5e754065159bdd5cb580eb9295f7256b2
-- 
2.53.0-Meta