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

[Frederic Weisbecker]

2011/9/26 Frederic Weisbecker <fweisbec@xxxxxxxxx>:
> 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.
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