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

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Date:	Thu, 19 May 2016 17:42:11 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Paolo Bonzini <pbonzini@...hat.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 05/19/2016 05:06 PM, Paolo Bonzini wrote:
> 
> 
> 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?

Right its cpu local. So a timer wakeup would be considered valid (if the timer
kicks in before the poll ends - the poll does also check if the timer expires and
maybe the hrtimer is a bit late.
> 
> Since it's not 4.7 material, I'll wait for your experiments and David's
> remarks.

I will try to get both patches scheduled but it might take a while.