linux-kernel - Re: [RFC PATCH] Fix abnormal rcu dynticks_nesting values related to async page fault

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Date:	Tue, 27 Nov 2012 23:53:07 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Li Zhong <zhong@...ux.vnet.ibm.com>,
	linux-next list <linux-next@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, paulmck@...ux.vnet.ibm.com,
	sasha.levin@...cle.com, avi@...hat.com
Subject: Re: [RFC PATCH] Fix abnormal rcu dynticks_nesting values related to
 async page fault

2012/11/27 Gleb Natapov <gleb@...hat.com>:
> What is the semantics of enter_idle()/exit_idle(), what are they used for?

It's used by drivers/idle/i7300_idle.c for some tracking. I don't know much
the details.

enter_idle() is called right before the CPU is set to lower power mode: hlt()

exit_idle() is called anytime we exit this low power mode: irq,
polling on need_resched(), etc...

> Not present fault happening in idle task does not mean we exit idle
> task. If this happens exception handler will execute sti; hlt waiting
> for missing page to be ready. Any interrupt happening during this hlt
> will do exit_idle() by itself.

Well, the code executed by this fault should be considered as an exit
from idle mode. Now it's a small piece of code and we hlt() quickly so
I guess getting of idle_exit() there wouldn't hurt much.

It's buggy for now anyway due to that race where we may not have an
opportunity to call back idle_enter() if the fault happens between the
idle_enter() in the idle loop and the hlt() in the idle loop.
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