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

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Message-ID: <20110926092052.GD18553@somewhere>
Date:	Mon, 26 Sep 2011 11:20:55 +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 06:26:11PM -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.

I believe it's always the case. tick_nohz_stop_sched_tick() is only called
from the first level of interrupt in irq_exit().

There is always some race window, as it's based on preempt offset: between
the sub_preempt_count and the softirqs begin and between softirqs end and the end
of the interrupt. But an "idle_cpu() || in_interrupt()" check in rcu_read_lock_sched_held()
should catch those offenders.
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