linux-kernel - RE: [v3 25/26] KVM: Suppress posted-interrupt when 'SN' is set

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Date:	Fri, 19 Dec 2014 23:34:51 +0000
From:	"Wu, Feng" <feng.wu@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	"Zhang, Yang Z" <yang.z.zhang@...el.com>,
	KVM list <kvm@...r.kernel.org>
CC:	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Wu, Feng" <feng.wu@...el.com>
Subject: RE: [v3 25/26] KVM: Suppress posted-interrupt when 'SN' is set



> -----Original Message-----
> From: Paolo Bonzini [mailto:paolo.bonzini@...il.com] On Behalf Of Paolo
> Bonzini
> Sent: Friday, December 19, 2014 8:01 PM
> To: Zhang, Yang Z; Wu, Feng; Paolo Bonzini; KVM list
> Cc: iommu@...ts.linux-foundation.org; linux-kernel@...r.kernel.org
> Subject: Re: [v3 25/26] KVM: Suppress posted-interrupt when 'SN' is set
> 
> 
> 
> On 19/12/2014 06:25, Zhang, Yang Z wrote:
> > I see your point. But from performance point, if we can schedule the
> > vCPU to another PCPU to handle the interrupt, it would helpful. But I
> > remember current KVM will not schedule the vCPU in run queue (even
> > though it got preempted) to another pCPU to run(Am I right?). So it
> > may hard to do it.
> 
> Yes.  If the vCPU is in the run queue, it means it exhausted its
> quantum.  As Feng said, the scheduler can decide to migrate it to
> another pCPU, or it can decide to leave it runnable but not start it.
> KVM doesn't try to force the scheduler one way or the other.
> 
> If the vCPU is I/O bound, it will not exhaust its quantum and will not
> be preempted.  It will block, and the wakeup vector will restart it.
> 
> I don't think urgent notifications are interesting.  If you want to do
> real time work, pin the vCPU to a physical CPU, and isolate the pCPU
> with isolcpus.  Then the vCPU will always be running.
> 
> Paolo

I agree, thanks Paolo!

Thanks,
Feng
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