linux-kernel - Re: linux-next-20110923: warning kernel/rcutree.c:1833

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Message-ID: <20110926223426.GO2399@linux.vnet.ibm.com>
Date:	Mon, 26 Sep 2011 15:34:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	linux-kernel@...r.kernel.org, Dipankar Sarma <dipankar@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Lai Jiangshan <laijs@...fujitsu.com>
Subject: Re: linux-next-20110923: warning kernel/rcutree.c:1833

On Mon, Sep 26, 2011 at 11:39:41AM +0200, Frederic Weisbecker wrote:
> On Sun, Sep 25, 2011 at 06:41:18PM -0700, Paul E. McKenney wrote:
> > On Sun, Sep 25, 2011 at 06:26:11PM -0700, Paul E. McKenney wrote:
> > > On Mon, Sep 26, 2011 at 03:10:33AM +0200, Frederic Weisbecker wrote:
> > > > 2011/9/26 Frederic Weisbecker <fweisbec@...il.com>:
> > > > > On Sun, Sep 25, 2011 at 09:48:04AM -0700, Paul E. McKenney wrote:
> > > > >> This is required for RCU_FAST_NO_HZ, which checks to see whether the
> > > > >> current CPU can accelerate the current grace period so as to enter
> > > > >> dyntick-idle mode sooner than it would otherwise.  This takes effect
> > > > >> in the situation where rcu_needs_cpu() sees that there are callbacks.
> > > > >> It then notes a quiescent state (which is illegal in an RCU read-side
> > > > >> critical section), calls force_quiescent_state(), and so on.  For this
> > > > >> to work, the current CPU must be in an RCU read-side critical section.
> > > > >
> > > > > You mean it must *not* be in an RCU read-side critical section (ie: in a
> > > > > quiescent state)?
> > > > >
> > > > > That assumption at least fails anytime in idle for the RCU
> > > > > sched flavour given that preemption is disabled in the idle loop.
> > > > >
> > > > >> If this cannot be made to work, another option is to call a new RCU
> > > > >> function in the case where rcu_needs_cpu() returned false, but after
> > > > >> the RCU read-side critical section has exited.
> > > > >
> > > > > You mean when rcu_needs_cpu() returns true (when we have callbacks
> > > > > enqueued)?
> > > > >
> > > > >> This new RCU function
> > > > >> could then attempt to rearrange RCU so as to allow the CPU to enter
> > > > >> dyntick-idle mode more quickly.  It is more important for this to
> > > > >> happen when the CPU is going idle than when it is executing a user
> > > > >> process.
> > > > >>
> > > > >> So, is this doable?
> > > > >
> > > > > At least not when we have RCU sched callbacks enqueued, given preemption
> > > > > is disabled. But that sounds plausible in order to accelerate the switch
> > > > > to dyntick-idle mode when we only have rcu and/or rcu bh callbacks.
> > > > 
> > > > But the RCU sched case could be dealt with if we embrace every use of
> > > > it with rcu_read_lock_sched() and rcu_read_unlock_sched(), or some light
> > > > version that just increases a local counter that rcu_needs_cpu() could check.
> > > > 
> > > > It's an easy thing to add: we can ensure preempt is disabled when we call it
> > > > and we can force rcu_dereference_sched() to depend on it.
> > > 
> > > Or just check to see if this is the first level of interrupt from the
> > > idle task after the scheduler is up.
> > 
> > Hmmm...  Is it the case that rcu_needs_cpu() gets called from within an
> > RCU read-side critical section only when called from an interrupt that
> > interrupted an RCU read-side critical section (keeping in mind that the
> > idle loop is a quiescent state regardless of preemption)?
> 
> Yeah. rcu_needs_cpu() can be called from an irq that either interrupted
> an rcu read side critical section or a bh one. But not a sched one if
> we forbid rcu sched uses in the preempt offset race windows I described
> in a previous mail.

But can't I just assume that if rcu_needs_cpu is invoked within
a second-level interrupt handler that it might be in any type of
RCU read-side critical section?  I could determine this by checking
RCU's dyntick-idle nesting state.

Such checks are not necessary if CONFIG_NO_HZ=n because in that
case rcu_needs_cpu() is just checking the callback queues, with
no assumptions about quiescent states.

> > If so, I should be able to do the appropriate checks within
> > rcu_needs_cpu().
> 
> Right. But to know if you interrupted an rcu read side, don't you
> need a specific counter when !CONFIG_PREEMPT?

Not if it is OK to assume that rcu_needs_cpu() can only be called from
within an RCU read-side interrupt handler if it is invoked from within a
second-level interrupt handler or if it interrupted some non-dyntick-idle
process-level code.

So, is this assumption valid?

							Thanx, Paul

> > The reason I didn't think of this earlier was that I thought that
> > rcu_needs_cpu() could be invoked from the idle notifier, which is itself
> > in an RCU read-side critical section.
> > 
> > 								Thanx, Paul
> 
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