Messages in this thread | | | Date | Mon, 26 Sep 2011 11:39:41 +0200 | From | Frederic Weisbecker <> | 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@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. > > 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)?
Yeah. rcu_needs_cpu() can be called from an irq that either interrupted an rcu read side critical section or a bh one. But not a sched one if we forbid rcu sched uses in the preempt offset race windows I described in a previous mail.
> > If so, I should be able to do the appropriate checks within > rcu_needs_cpu().
Right. But to know if you interrupted an rcu read side, don't you need a specific counter when !CONFIG_PREEMPT?
> The reason I didn't think of this earlier was that I thought that > rcu_needs_cpu() could be invoked from the idle notifier, which is itself > in an RCU read-side critical section. > > Thanx, Paul -- 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/
| |