* Pekka Enberg <penberg@xxxxxxxxxx> wrote: > On Thu, May 26, 2011 at 9:11 PM, Avi Kivity <avi@xxxxxxxxxx> wrote: > > On 05/26/2011 09:05 PM, Ingo Molnar wrote: > >> > >> > > >> > I've added some rwlocks because of what Ingo said yesterday about > >> > adding/removing devices after the first initialization phase. > >> > > >> > Take MMIO lock for example: Since we can now run SMP guests, we may > >> > have multiple MMIO exits (one from each VCPU thread). Each of those > >> > exits leads to searching the MMIO rbtree. > >> > > >> > We can use a mutex to lock it, but it just means that those threads > >> > will be blocked there instead of concurrently searching the MMIO > >> > tree which makes the search linear instead of parallel. > >> > > >> > It's hard to bring 'real' numbers at this stage because the only > >> > 'real' device we have which uses MMIO is the VESA driver, and we > >> > can't really simulate many VCPUs writing to it :) > >> > >> I'd suggest keeping it simple first - rwlocks are nasty and will > >> bounce a cacheline just as much. > > > > Well, this is the first case where tools/kvm can do better than qemu with > > its global lock, so I think it's worth it. > > > >> If lookup scalability is an issue we can extend RCU to tools/kvm/. > > > > Definitely rcu is a perfect patch for mmio dispatch. > > Userspace RCU code is here, Sasha, if you feel like tackling this: > > http://lttng.org/urcu > > :-) > > I'm CC'ing Paul and Mathieu as well for urcu. I think we should rather share some of the kernel RCU code in an intelligent way instead of bringing in yet another library which is a IIRC a distant copy of the kernel code to begin with. That way we could lazily benefit from all the enhancements Paul does to the kernel RCU code! :-) Note that kernel/treercu.c is pretty tied to the kernel right now, so a first approach could be to: - Check Paul's excellent documentation about RCU and make sure you don't miss Paul's excellent 3-part primer on LWN.net: http://lwn.net/Articles/262464/ http://lwn.net/Articles/263130/ http://lwn.net/Articles/264090/ There are also lots of very good RCU articles on the LWN Kernel Index page: http://lwn.net/Kernel/Index/ - Check kernel/tinyrcu.c to see how RCU is implemented in its simplest form. :) - Copy the tree-RCU code from kernel/treercu.c to tools/kvm/rcu/ - Massage it so far that it is suitable for tools/kvm/. We really only need a few core RCU facilities initially: struct rcu_head; rcu_read_lock(); rcu_read_unlock(); rcu_dereference() call_rcu(head, func); rcu_synchronize(); The rest, _bh(), etc. are all kernelisms. - Then once it's working we could look at doing the code sharing *for real*: splitting the functionality out of the original treercu.c code into kernel/treercu-lib.c and rcuupdate-lib.h or so and include that one in tools/kvm/rcu/. - [ You might also benefit from porting rcutorture code to user-space. It will catch RCU bugs like nothing else. ] That way the 'core RCU' logic would be contained in treercu-lib.c, all kernel glue would be in kernel/rcutree.c. Some other approach might be possible as well, this was just a first rough idea :) Thanks, Ingo -- 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
