[PATCH v2] sched/fair: Document the slow path and fast path in select_task_rq_fair

[Barry Song]

From: Barry Song <song.bao.hua@xxxxxxxxxxxxx>

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.

Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
---
 -v2: refine according to Steven's comments, thanks!

 kernel/sched/fair.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f6a05d9b5443..816c8ddf1b6d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6951,6 +6951,11 @@ 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)
-- 
2.25.1