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Subject[tip: sched/core] sched/fair: Document the slow path and fast path in select_task_rq_fair
The following commit has been merged into the sched/core branch of tip:

Commit-ID: 2917406c352757642c3c1a13a4c99c96e6d22fde
Gitweb: https://git.kernel.org/tip/2917406c352757642c3c1a13a4c99c96e6d22fde
Author: Barry Song <song.bao.hua@hisilicon.com>
AuthorDate: Sat, 16 Oct 2021 19:11:09 +08:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 07 Dec 2021 15:14:10 +01:00

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 | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0672218..f34f2f3 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)
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