linux-kernel - Re: [PATCH] KVM: add halt_attempted

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Message-ID: <55F9413A.8000006@redhat.com>
Date:	Wed, 16 Sep 2015 12:15:22 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	rkrcmar@...hat.com, David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	David Matlack <dmatlack@...gle.com>,
	Jens Freimann <jfrei@...ux.vnet.ibm.com>
Subject: Re: [PATCH] KVM: add halt_attempted_poll to VCPU stats



On 16/09/2015 12:12, Christian Borntraeger wrote:
> I am looking into a case right now, where auto polling goes 
> completely nuts on my system:
> 
> guest1: 8vcpus		guest2: 1 vcpu
> iperf with 25 process (-P25) from guest1 to guest2.
> 
> I/O interrupts on s390 are floating (pending on all CPUs) so on 
> ALL VCPUs that go to sleep, polling will consider any pending
> network interrupt as successful poll. So with auto polling the
> guest consumes up to 5 host CPUs without auto polling only 1.
> Reducing  halt_poll_ns to 100000 seems to work (goes back to 
> 1 cpu).
> 
> The proper way might be to feedback the result of the
> interrupt dequeue into the heuristics. Don't know yet how
> to handle that properly.

I think it's simplest to disable halt_poll_ns by default on s390.  On
x86, for example, you can mark interrupts so that they _can_ be
delivered to all CPUs but only one will get it.

You can add a Kconfig symbol for that to other architectures, and not s390.

Paolo
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