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

JIRA: https://issues.redhat.com/browse/RHEL-78821

commit 8e113df990c9df70fc6d83ebd53ee1b2867c23c4
Author: Zhongqiu Han <quic_zhonhan@quicinc.com>
Date:   Wed Oct 9 17:37:45 2024 +0800

    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 783e391b7b ("x86: Simplify
    cpu_idle_wait"), rmb() no longer has a reason to exist.

    After that, commit d166991234 ("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 cf37b6b484 ("sched/idle: Move cpu/idle.c to
    sched/idle.c") moved cpu/idle.c to sched/idle.c. And commit c1de45ca83
    ("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

Signed-off-by: Phil Auld <pauld@redhat.com>
This commit is contained in:
Phil Auld 2025-02-12 15:52:17 +00:00
parent 7da0049308
commit e1f44a3075
1 changed files with 0 additions and 1 deletions
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1
kernel/sched/idle.c
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@ -250,7 +250,6 @@ static void do_idle(void)
tick_nohz_idle_enter();
while (!need_resched()) {
rmb();
/*
* Interrupts shouldn't be re-enabled from that point on until