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

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Message-ID: <20121127165958.GI2474@linux.vnet.ibm.com>
Date:	Tue, 27 Nov 2012 08:59:58 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Gleb Natapov <gleb@...hat.com>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	linux-next list <linux-next@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, sasha.levin@...cle.com,
	avi@...hat.com
Subject: Re: [RFC PATCH] Fix abnormal rcu dynticks_nesting values related to
 async page fault

On Tue, Nov 27, 2012 at 05:48:18PM +0100, Frederic Weisbecker wrote:
> 2012/11/27 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> > On Tue, Nov 27, 2012 at 04:56:30PM +0100, Frederic Weisbecker wrote:
> >> 2012/11/27 Gleb Natapov <gleb@...hat.com>:
> >> > For KVM_PV_REASON_PAGE_NOT_PRESENT it behaves like an exception.
> >>
> >> Ok.
> >> There seem to be a bug in kvm_async_pf_task_wait(). Using
> >> idle_cpu(cpu) to find out if the current task is the idle task may not
> >> work if there is pending wake up. Me may schedule another task but
> >> when that other task sleeps later we can't schedule back to idle until
> >> the fault is completed.
> >>
> >> The right way is to use is_idle_task(current)
> >
> > Agreed.  But if you instead need to figure out whether or not RCU believes
> > that the CPU is idle, use rcu_is_cpu_idle().  You need to figure out
> > whether or not RCU believes that the CPU is idle if you are trying to
> > play fast and loose with rcu_irq_enter() and rcu_irq_exit().
> 
> Right. I was referring to another problem though. They don't want to
> schedule on the following cases:
> 
> 1) preemption disabled, of course.
> 3) we are in the idle task. Because if we want to sleep in order to
> wait for the page fault completion, we need to be able to schedule the
> idle task. But we are already the idle task.
> 
> Now idle_cpu(cpu) is the wrong check for that.

Fair enough!  ;-)

							Thanx, Paul

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