linux-kernel - Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will fire soon

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Message-ID: <83e82f6b-bc6b-a85b-e2c7-9a1b4d4997d1@redhat.com>
Date:	Thu, 19 May 2016 17:06:33 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>,
	Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will
 fire soon



On 19/05/2016 17:03, Christian Borntraeger wrote:
> > > Would this work too and be simpler?
> > > Hmm, your patch does only fiddle with the grow/shrink logic (which might
> > > be a good idea independently of this change), but the original patch 
> > > actually takes into account that we have a guaranteed maximum time by a 
> > > wakeup timer - IOW we know exactly what the maximum poll time is.
> > 
> > Yes, it's different.  The question is whether a 10us poll (40,000 clock
> > cycles) has an impact even if it's sometimes wrong.
> 
> Valid question. As I said, this change might be something good independent from
> the original patch. (it might make it unnecessary, though) On the other hand
> I can handle ~30 guest entry/exit cycles of a simple exit like diag9c.
> Needs measurement. 

Actually I'm okay with the original patch, and especially on s390 where
the maximum poll time is small it may make a bigger difference.  Though
I suppose the timer interrupt is not floating?

Since it's not 4.7 material, I'll wait for your experiments and David's
remarks.

Paolo