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

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Date:	Thu, 3 Jun 2010 10:52:51 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Srivatsa Vaddagiri <vatsa@...ibm.com>
Cc:	Avi Kivity <avi@...hat.com>, Andi Kleen <andi@...stfloor.org>,
	Gleb Natapov <gleb@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, hpa@...or.com, mingo@...e.hu, npiggin@...e.de,
	tglx@...utronix.de, mtosatti@...hat.com
Subject: Re: [PATCH] use unfair spinlock when running on hypervisor.

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@...ux.intel.com -- Speaking for myself only.
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