sched/fair: Document the slow path and fast path in select_task_rq_fair

All People I know including myself took a long time to figure out that
typical wakeup will always go to fast path and never go to slow path
except WF_FORK and WF_EXEC.

Vincent reminded me once in a linaro meeting and made me understand
slow path won't happen for WF_TTWU. But my other friends repeatedly
wasted a lot of time on testing this path like me before I reminded
them.

So obviously the code needs some document.

Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20211016111109.5559-1-21cnbao@gmail.com

kernel/sched/fair.c

@@ -6917,6 +6917,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 			break;
 		}
 
+		/*
+		 * Usually only true for WF_EXEC and WF_FORK, as sched_domains
+		 * usually do not have SD_BALANCE_WAKE set. That means wakeup
+		 * will usually go to the fast path.
+		 */
 		if (tmp->flags & sd_flag)
 			sd = tmp;
 		else if (!want_affine)