On Sun, May 29, 2011 at 06:00:00PM +0300, Avi Kivity wrote: > On 05/29/2011 05:27 PM, Ingo Molnar wrote: > >* Avi Kivity<avi@xxxxxxxxxx> wrote: > > > >> I don't understand how you expect per_cpu to work in userspace. As > >> soon as you calculate the per-cpu address, it can be invalidated. > >> It doesn't help that you get a signal; you've already calculated > >> the address. > > > >I was thinking of some sort of transactional mechanism, a tightly > >controlled set of assembly instructions updating the percpu fields, > >where the migration event would be able to 'unroll' incomplete > >modifications done to the 'wrong' percpu data structure. (It would be > >rather complex and every percpu op would have to be an atomic because > >there's always the chance that it's executed on the wrong CPU.) > > > >But note that we do not even need any notification if there's a > >(local) lock on the percpu fields: > > > >It will work because it's statistically percpu the lock will not > >SMP-bounce between CPUs generally so it will be very fast to > >acquire/release it, and we get the full cache benefits of percpu > >variables. > > > >The migration notification would still be useful to detect grace > >periods at natural points - but as Paul pointed out polling it via > >SIGALRM works as well. The two (migration and SIGALRM) could be > >combined as well. > > I think it's way simpler to map cpu == thread. And in fact, when > you run a Linux kernel in a kvm guest, that's what happens, since > each vcpu _is_ a host thread. I have to agree with Avi here. If a stop_machine()-like approach is going to work, the updates have to be very rare, so any additional cache-nonlocality from having lots of threads should not be a problem. Especially given that in this particular case, there are exactly as many CPUs as threads anyway. The readers should only need to touch a constant number of cache lines either way. Or am I missing something here? Thanx, Paul -- 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
