Message-ID: <20110926094203.GF18553@somewhere>
Date:	Mon, 26 Sep 2011 11:42:06 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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)?
> 
> If so, I should be able to do the appropriate checks within
> rcu_needs_cpu().

It sounds better to me if you can do all the checks from rcu_needs_cpu()
so that all you need is to wait for another jiffy to escape the read side
critical section.

Doing something from the read side exit path would require some weird
trickiness.
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