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On Tue, May 10, 2005 at 10:08:11PM +0200, Peter Zijlstra wrote: > 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. But the flip is an integral part of detecting a grace period. So, if I understand your proposal correctly, I would have to flip to figure out when it was safe to flip. What am I missing here? 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/ | ||||||||||||
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