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

[Ingo Molnar <mingo@elte.hu>]

* Sasha Levin <levinsasha928@gmail.com> wrote:

> On Thu, 2011-05-26 at 19:02 +0300, Pekka Enberg wrote:
> > On Thu, 26 May 2011, Sasha Levin wrote:
> > > Adds a rwlock wrapper which like the mutex wrapper makes rwlock calls
> > > similar to their kernel counterparts.
> > >
> > > Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
> > 
> > There's no explanation why a mutex isn't sufficient. The pthread 
> > locking primitives aren't all that great in practice so unless 
> > you have some correctness issue that requires a rwlock or some 
> > numbers, I'd prefer you go for a mutex.
> 
> 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.

If lookup scalability is an issue we can extend RCU to tools/kvm/.

Thanks,

	Ingo