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

Paul E. McKenney
2011-05-30T03:07:32Z

On Sun, May 29, 2011 at 09:33:27PM +0200, Ingo Molnar wrote:
>
> * 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!
> :-)

Ah! I did miss a turn in the dicussion, then. ;-)

My initial thought is to start with CPU==thread, with the CPU online
and offline operations used at thread creation and destruction time.

But longer term, you are right that there would be cache-locality
benefits to splitting. Perhaps more important, user-space testing
could then achieve much better coverage of the various race conditions.

> 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 :-)

Makes sense!

The other debugging use for the read-side primitives is to execute
the read-side ready-do-respond-to-kvm-pause code. This can help
catch bugs where the developer put the br_read_lock() and the
br_read_unlock() in the wrong place.

Thanx, Paul