[PATCH] sched_ext: Separate lock and first_task into distinct cache lines in scx_dispatch_q

[David Carlier <devnexen@gmail.com>]

lock (write-heavy) and first_task (read-mostly, lockless RCU peek) share
the same cache line in struct scx_dispatch_q. Every lock acquire/release
by a dispatching CPU invalidates the line for all CPUs performing
lockless first_task peeks, causing unnecessary cache coherence traffic,
especially across NUMA nodes.

Add ____cacheline_aligned_in_smp to first_task to place it on its own
cache line, eliminating this false sharing on SMP systems. On
uniprocessor builds the annotation is a no-op, so no space is wasted.

On SMP, the trade-off is increased struct size: each scx_dispatch_q
grows by up to ~56 bytes of padding. There are two instances embedded
per-CPU in scx_rq (local_dsq and bypass_dsq), plus any dynamically
allocated custom DSQs, so the total overhead scales with the number of
CPUs and active DSQs.

Signed-off-by: David Carlier <devnexen@gmail.com>
---
 include/linux/sched/ext.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index bcb962d5ee7d..2988df68a97a 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -70,7 +70,7 @@ enum scx_dsq_id_flags {
  */
 struct scx_dispatch_q {
 	raw_spinlock_t		lock;
-	struct task_struct __rcu *first_task; /* lockless peek at head */
+	struct task_struct __rcu *first_task ____cacheline_aligned_in_smp; /* lockless peek at head */
 	struct list_head	list;	/* tasks in dispatch order */
 	struct rb_root		priq;	/* used to order by p->scx.dsq_vtime */
 	u32			nr;
-- 
2.51.0