[PATCH] 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 understand typical
wake_up will always go to fast path and never go to slow path except
FORK and EXEC.
Vincent reminded me once in a linaro meeting and it 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>
---
 kernel/sched/fair.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f6a05d9b5443..ccf8f73dd4d1 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;
 		}
 
+		/*
+		 * This is usually true only for WF_EXEC and WF_FORK, for WF_TTWU
+		 * it is almost always false as sched_domain hasn't SD_BALANCE_WAKE
+		 * in default
+		 */
 		if (tmp->flags & sd_flag)
 			sd = tmp;
 		else if (!want_affine)
@@ -6958,7 +6963,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
 	}
 
 	if (unlikely(sd)) {
-		/* Slow path */
+		/*
+		 * Slow path, usually only for WF_EXEC and WF_FORK; WF_TTWU almost
+		 * always goes to fast path as sched_domain hasn't SD_BALANCE_WAKE
+		 * in default
+		 */
 		new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag);
 	} else if (wake_flags & WF_TTWU) { /* XXX always ? */
 		/* Fast path */
-- 
2.25.1