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On Mon, 2005-05-09 at 18:24 -0700, Paul E. McKenney wrote: > > Counter-Based Approach> > The current implementation in Ingo's CONFIG_PREEMPT_RT patch uses a > counter-based approach, which seems to work, but which can result in > indefinite-duration grace periods. The following are very hazy thoughts > on how to get the benefits of this approach, but with short grace periods. > > 1. The basic trick is to maintain a pair of counters per CPU. > There would also be a global boolean variable that would select > one or the other of each pair. The rcu_read_lock() primitive > would then increment the counter indicated by the boolean > corresponding to the CPU that it is currently running on. > It would also keep a pointer to that particular counter in > the task structure. The rcu_read_unlock() primitive would > decrement this counter. (And, yes, you would also have a > counter in the task structure so that only the outermost of > a set of nested rcu_read_lock()/rcu_read_unlock() pairs would > actually increment/decrement the per-CPU counter pairs.) > > To force a grace period, one would invert the value of the > global boolean variable. Once all the counters indicated > by the old value of the global boolean variable hit zero, > the corresponding set of RCU callbacks can be safely invoked. > > The big problem with this approach is that a pair of inversions > of the global boolean variable could be spaced arbitrarily > closely, especially when you consider that the read side code > can be preempted. This could cause RCU callbacks to be invoked > prematurely, which could greatly reduce the life expectancy > of your kernel. > Thoughts?> How about having another boolean indicating the ability to flip the selector boolean. This boolean would be set false on an actual flip and cleared during a grace period. That way the flips cannot ever interfere with one another such that the callbacks would be cleared prematurely. -- Peter Zijlstra <a.p.zijlstra@chello.nl> - 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/ | ||||||||||
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