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@quicinc.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 d2f096bb274c..ab911d1335ba 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

On Wed, Oct 09, 2024 at 05:37:45PM +0800, Zhongqiu Han wrote:
> 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@quicinc.com>
> History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git

Thanks!

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@quicinc.com>
AuthorDate:    Wed, 09 Oct 2024 17:37:45 +08:00
Committer:     Peter Zijlstra <peterz@infradead.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@quicinc.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.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