Re: [PATCH 4/6] kvm tools: Add rwlock wrapper

[Ingo Molnar <mingo@elte.hu>]

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> 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@redhat.com>  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?

I was talking about the (still imaginery!) user-space tree-RCU code!
:-)

The stop_machine_run()-alike thing is for brlocks - for which Sasha 
sent patches already, see these patches on the kvm@vger.kernel.org 
list:

   [PATCH 3/4] kvm tools: Add a brlock
   [PATCH 4/4] kvm tools: Use brlock in MMIO and IOPORT

Wrt. brlocks, we'll keep them as simple as possible and indeed no 
involved tricks are needed AFAICS. read_lock() will be a compiler 
barrier(), that's as fast as it gets :-)

Thanks,

	Ingo