From 2917406c352757642c3c1a13a4c99c96e6d22fde Mon Sep 17 00:00:00 2001 From: Barry Song Date: Sat, 16 Oct 2021 19:11:09 +0800 Subject: 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 Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20211016111109.5559-1-21cnbao@gmail.com --- kernel/sched/fair.c | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'kernel') diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 06722188df49..f34f2f344fe9 100644 --- a/kernel/sched/fair.c +++ b/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) -- cgit v1.2.3-70-g09d2