From mboxrd@z Thu Jan  1 00:00:00 1970
From: Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH 3/4] arm64: KVM: let other tasks run when hitting WFE
Date: Mon, 22 Jul 2013 14:01:18 +0100
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Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
 Marc Zyngier <Marc.Zyngier@arm.com>,
 Raghavendra KT <raghavendra.kt.linux@gmail.com>,
 Raghavendra KT <raghavendra.kt@linux.vnet.ibm.com>,
 Catalin Marinas <Catalin.Marinas@arm.com>,
 "kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
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To: Christoffer Dall <christoffer.dall@linaro.org>
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On Mon, Jul 22, 2013 at 01:51:52PM +0100, Christoffer Dall wrote:
> On 22 July 2013 10:53, Raghavendra KT <raghavendra.kt.linux@gmail.com> wrote:
> > On Fri, Jul 19, 2013 at 7:23 PM, Marc Zyngier <marc.zyngier@arm.com> wrote:
> >> So far, when a guest executes WFE (like when waiting for a spinlock
> >> to become unlocked), we don't do a thing and let it run uninterrupted.
> >>
> >> Another option is to trap a blocking WFE and offer the opportunity
> >> to the scheduler to switch to another task, potentially giving the
> >> vcpu holding the spinlock a chance to run sooner.
> >>
> >
> > Idea looks to be correct from my experiments on x86. It does bring some
> > percentage of benefits in overcommitted guests. Infact,
> >
> > https://lkml.org/lkml/2013/7/22/41 tries to do the same thing for x86.
> > (this results in using ple handler heuristics in vcpu_block pach).
> 
> What about the adverse effect in the non-overcommitted case?

You might hope to reduce overall system latency, in the same way that
PREEMPT kernels implement spin_lock in terms of spin_trylock. Marc might
have some ideas about the practical performance of this, but he's currently
on holiday and (assumedly) not reading the list!

Will