Date: Thu, 3 Jun 2010 10:52:51 +0200
From: Andi Kleen <andi@firstfloor.org>
To: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: Avi Kivity <avi@redhat.com>, Andi Kleen <andi@firstfloor.org>,
       Gleb Natapov <gleb@redhat.com>, linux-kernel@vger.kernel.org,
       kvm@vger.kernel.org, hpa@zytor.com, mingo@elte.hu, npiggin@suse.de,
       tglx@linutronix.de, mtosatti@redhat.com
Subject: Re: [PATCH] use unfair spinlock when running on hypervisor.
Message-ID: <20100603085251.GA4166@basil.fritz.box>
References: <20100601093515.GH24302@redhat.com> <87sk56ycka.fsf@basil.nowhere.org> <20100601162414.GA6191@redhat.com> <20100601163807.GA11880@basil.fritz.box> <4C053ACC.5020708@redhat.com> <20100601172730.GB11880@basil.fritz.box> <4C05C722.1010804@redhat.com> <20100602085055.GA14221@basil.fritz.box> <4C061DAB.6000804@redhat.com> <20100603042051.GA5953@linux.vnet.ibm.com>
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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.
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