* Ingo Molnar (mingo@xxxxxxx) wrote: > > * Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote: > > > > - Check kernel/tinyrcu.c to see how RCU is implemented in its > > > simplest form. :) > > > > ...so simplistic it only works on UP systems, which are not so common > > these days on the systems targeted by kvm. > > As i said above, in its simplest form - which is UP. > > Obviously it's not tinyrcu.c that should be used by tools/kvm/ but > what i suggested, tree-RCU: I agree that tree-RCU has some grace-period management scalability benefits that would be interesting to have. > > > - Copy the tree-RCU code from kernel/treercu.c to tools/kvm/rcu/ > > > > This code is very much tied with the kernel scheduler. [...] > > It would not be particularly complex to enable user-space to request > a callback on context switch events. > > I was thinking on and off about allowing perf events to generate a > per sampling event notification signal on specific events, such as > page faults or context switches. > > Obviously this won't be enabled from NMI contexts due to atomicity > constraints, but the pagefault and maybe the context switch path > looks doable. > > That capability would be a rather simple kernel change and it would > allow a user-space RCU implementation to be notified of various key > events, context switches in particular. I'm worried about "self-recursion" behaviors that could be triggered though: if the userland callback code called from a page fault triggers a page fault all by itself, then it looks like a good way to bring the system to its knees. The same apply to context switches. Do you have a way to handle this in mind ? > > Would you be interested in helping code up such a facility? The urcu > library could make good use of it i think, regardless of what we do > in tools/kvm/. I'm open to try finding out the best possible approach to support RCU in user-space (disclaimer: I might need some help on this due to my time being fully taken by contracts currently). I guess the sweet spot will end up being at a crossroad between kernel-only and userland-only solution. Thanks, Mathieu > > Thanks, > > Ingo -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
