Re: [PATCH] use unfair spinlock when running on hypervisor.

Andi Kleen
2010-06-03T08:52:58Z

On Thu, Jun 03, 2010 at 09:50:51AM +0530, Srivatsa Vaddagiri wrote:
> On Wed, Jun 02, 2010 at 12:00:27PM +0300, Avi Kivity wrote:
> >
> > There are two separate problems: the more general problem is that
> > the hypervisor can put a vcpu to sleep while holding a lock, causing
> > other vcpus to spin until the end of their time slice. This can
> > only be addressed with hypervisor help.
>
> Fyi - I have a early patch ready to address this issue. Basically I am using
> host-kernel memory (mmap'ed into guest as io-memory via ivshmem driver) to hint
> host whenever guest is in spin-lock'ed section, which is read by host scheduler
> to defer preemption.

Looks like a ni.ce simple way to handle this for the kernel.

However I suspect user space will hit the same issue sooner
or later. I assume your way is not easily extensable to futexes?

> One pathological case where this may actually hurt is routines in guest like
> flush_tlb_others_ipi() which take a spinlock and then enter a while() loop
> waiting for other cpus to ack something. In this case, deferring preemption just
> because guest is in critical section actually hurts! Hopefully the upper bound
> for deferring preemtion and the fact that such routines may not be frequently
> hit should help alleviate such situations.

So do you defer during the whole spinlock region or just during the spin?

I assume the the first?

-Andi
--
ak@linux.intel.com -- Speaking for myself only.