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

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Message-ID: <20240802143007.2438213-1-quic_zhonhan@quicinc.com>
Date: Fri, 2 Aug 2024 22:30:07 +0800
From: Zhongqiu Han <quic_zhonhan@...cinc.com>
To: <mingo@...hat.com>, <peterz@...radead.org>, <juri.lelli@...hat.com>,
        <vincent.guittot@...aro.org>, <dietmar.eggemann@....com>,
        <rostedt@...dmis.org>, <bsegall@...gle.com>, <vschneid@...hat.com>,
        <mgorman@...e.de>
CC: <linux-kernel@...r.kernel.org>, <quic_zhonhan@...cinc.com>
Subject: [PATCH] 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().

Signed-off-by: Zhongqiu Han <quic_zhonhan@...cinc.com>
History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git
---
 kernel/sched/idle.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index e53e2da04ba4..712c1ce16c8b 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
-- 
2.25.1