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

Paul E. McKenney
2011-09-26T01:26:18Z

On Mon, Sep 26, 2011 at 03:10:33AM +0200, Frederic Weisbecker wrote:
> 2011/9/26 Frederic Weisbecker <fweisbec@gmail.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.

Thanx, Paul