linux-kernel - [tip: sched/core] sched: idle: Optimize the generic idle loop by removing needless memory barrier

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Message-ID: <172923526546.1442.670768403460396409.tip-bot2@tip-bot2>
Date: Fri, 18 Oct 2024 07:07:45 -0000
From: "tip-bot2 for Zhongqiu Han" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Zhongqiu Han <quic_zhonhan@...cinc.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched: idle: Optimize the generic idle loop by
 removing needless memory barrier

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     8e113df990c9df70fc6d83ebd53ee1b2867c23c4
Gitweb:        https://git.kernel.org/tip/8e113df990c9df70fc6d83ebd53ee1b2867c23c4
Author:        Zhongqiu Han <quic_zhonhan@...cinc.com>
AuthorDate:    Wed, 09 Oct 2024 17:37:45 +08:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Mon, 14 Oct 2024 12:52:40 +02:00

sched: idle: Optimize the generic idle loop by removing needless memory barrier

The memory barrier rmb() in generic idle loop do_idle() function is not
needed, it doesn't order any load instruction, just remove it as needless
rmb() can cause performance impact.

The rmb() was introduced by the tglx/history.git commit f2f1b44c75c4
("[PATCH] Remove RCU abuse in cpu_idle()") to order the loads between
cpu_idle_map and pm_idle. It pairs with wmb() in function cpu_idle_wait().

And then with the removal of cpu_idle_state in function cpu_idle() and
wmb() in function cpu_idle_wait() in commit 783e391b7b5b ("x86: Simplify
cpu_idle_wait"), rmb() no longer has a reason to exist.

After that, commit d16699123434 ("idle: Implement generic idle function")
implemented a generic idle function cpu_idle_loop() which resembles the
functionality found in arch/. And it retained the rmb() in generic idle
loop in file kernel/cpu/idle.c.

And at last, commit cf37b6b48428 ("sched/idle: Move cpu/idle.c to
sched/idle.c") moved cpu/idle.c to sched/idle.c. And commit c1de45ca831a
("sched/idle: Add support for tasks that inject idle") renamed function
cpu_idle_loop() to do_idle().

History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
Signed-off-by: Zhongqiu Han <quic_zhonhan@...cinc.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20241009093745.9504-1-quic_zhonhan@quicinc.com
---
 kernel/sched/idle.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index d2f096b..ab911d1 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -271,7 +271,6 @@ static void do_idle(void)
 	tick_nohz_idle_enter();

 	while (!need_resched()) {
-		rmb();

 		/*
 		 * Interrupts shouldn't be re-enabled from that point on until