sched_ext: Remove unnecessary cpu_relax()

As described in commit b07996c ("sched_ext: Don't hold
scx_tasks_lock for too long"), we're doing a cond_resched() every 32
calls to scx_task_iter_next() to avoid RCU and other stalls. That commit
also added a cpu_relax() to the codepath where we drop and reacquire the
lock, but as Waiman described in [0], cpu_relax() should only be
necessary in busy loops to avoid pounding on a cacheline (or to allow a
hypertwin to more fully utilize a core).

Let's remove the unnecessary cpu_relax().

[0]: https://lore.kernel.org/all/[email protected]/

Cc: Waiman Long <[email protected]>
Signed-off-by: David Vernet <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: Byte-Lab

kernel/sched/ext.c

@@ -1374,7 +1374,6 @@ static struct task_struct *scx_task_iter_next(struct scx_task_iter *iter)
 
 	if (!(++iter->cnt % SCX_OPS_TASK_ITER_BATCH)) {
 		scx_task_iter_unlock(iter);
-		cpu_relax();
 		cond_resched();
 		scx_task_iter_relock(iter);
 	}