linux-kernel - [tip: sched/core] sched/fair: Document the slow path and fast path in select_task_rq

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Date:   Tue, 07 Dec 2021 14:22:17 -0000
From:   "tip-bot2 for Barry Song" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Barry Song <song.bao.hua@...ilicon.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
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@...ilicon.com>
AuthorDate:    Sat, 16 Oct 2021 19:11:09 +08:00
Committer:     Peter Zijlstra <peterz@...radead.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@...ilicon.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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)