Why we need to call cpu_idle() with preemption disabled

From: Wu Zhangjin <wuzhangjin@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Steven Rostedt <srostedt@redhat.com>,
	linux-kernel <linux-kernel@vger.kernel.org>
Subject: Why we need to call cpu_idle() with preemption disabled
Date: Tue, 16 Mar 2010 17:01:54 +0800	[thread overview]
Message-ID: <1268730114.9552.34.camel@falcon> (raw)

Hi, Thomas

Just traced the preemption latency of 2.6.33-rt7 on my Yeeloong netbook
with the preemptoff tracer of Ftrace and found it is very big in
cpu_idle(), more than 1000 us.

And found that we have called cpu_idle() in init/main.c with preemption
disabled? why we need to do it? can we simply call it with preemption
enabled?

diff --git a/init/main.c b/init/main.c
index 48393c0..437ac34 100644
--- a/init/main.c
+++ b/init/main.c
@@ -428,9 +428,8 @@ static noinline void __init_refok rest_init(void)
         */
        init_idle_bootup_task(current);
        preempt_enable_and_schedule();
-       preempt_disable();
 
-       /* Call into cpu_idle with preempt disabled */
+       /* There is no reason for calling cpu_idle with preemption
disabled */
        cpu_idle();
 }

After removing that preempt_disable() and the related operations around
the calling to __schedule() in the cpu_idle(), the result becomes around
200 us, which is acceptable for I have enabled several Ftrace tracers.

Best Regards,
	Wu Zhangjin

