Messages in this thread | | | Date | Mon, 26 Sep 2011 03:10:33 +0200 | Subject | Re: linux-next-20110923: warning kernel/rcutree.c:1833 | From | Frederic Weisbecker <> |
| |
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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |