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 19:12:40 +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>:
> On Tue, Nov 27, 2012 at 06:30:32PM +0100, Frederic Weisbecker wrote:
>> 2012/11/27 Gleb Natapov <gleb@...hat.com>:
>> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> > index 4180a87..636800d 100644
>> > --- a/arch/x86/kernel/kvm.c
>> > +++ b/arch/x86/kernel/kvm.c
>> > @@ -113,7 +113,7 @@ void kvm_async_pf_task_wait(u32 token)
>> >         int cpu, idle;
>> >
>> >         cpu = get_cpu();
>> > -       idle = idle_cpu(cpu);
>> > +       idle = is_idle_task(current);
>>
>> I suggest this part goes to a standalone patch.
>>
>> >         put_cpu();
>> >
>> >         spin_lock(&b->lock);
>> > @@ -247,10 +247,7 @@ do_async_page_fault(struct pt_regs *regs, unsigned long error_code)
>> >                 break;
>> >         case KVM_PV_REASON_PAGE_NOT_PRESENT:
>> >                 /* page is swapped out by the host. */
>> > -               rcu_irq_enter();
>> > -               exit_idle();
>> >                 kvm_async_pf_task_wait((u32)read_cr2());
>> > -               rcu_irq_exit();
>>
>> Hmm, we still need those above around. I believe we just need to add
>> rcu_user_exit() in the beginning of that case.
> The exception may happen in kernel space too. Is calling rcu_user_exit()
> still OK? Also why calling exit_idle() if we are not exiting idle?

Yeah, rcu_user_exit() takes care of that. And exit_idle() also checks
we are really idle before firing the notifier.

Now we should probably call back enter_idle() before resuming idle if
needed. We disable irqs before calling enter_idle(). And exit_idle()
is called from irqs. This way we ensure it's either called before we
called local_irq_disable() or while the CPU is halt(). This provides
the guarantee that enter_idle() is always called before the CPU goes
to sleep. The fact we call exit_idle()  from an exception in idle
breaks this guarantee. But that's another issue.
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