linux-kernel - Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

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Date:	Thu, 11 Jul 2013 12:48:33 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	Andrew Jones <drjones@...hat.com>, mingo@...hat.com,
	ouyang@...pitt.edu, habanero@...ux.vnet.ibm.com, jeremy@...p.org,
	x86@...nel.org, konrad.wilk@...cle.com, hpa@...or.com,
	pbonzini@...hat.com, linux-doc@...r.kernel.org,
	xen-devel@...ts.xensource.com, peterz@...radead.org,
	mtosatti@...hat.com, stefano.stabellini@...citrix.com,
	andi@...stfloor.org, attilio.rao@...rix.com, gregkh@...e.de,
	agraf@...e.de, chegu_vinod@...com, torvalds@...ux-foundation.org,
	avi.kivity@...il.com, tglx@...utronix.de, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, riel@...hat.com,
	virtualization@...ts.linux-foundation.org,
	srivatsa.vaddagiri@...il.com
Subject: Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

On Thu, Jul 11, 2013 at 02:43:03PM +0530, Raghavendra K T wrote:
> On 07/10/2013 04:03 PM, Gleb Natapov wrote:
> [...] trimmed
> 
> >>>Yes. you are right. dynamic ple window was an attempt to solve it.
> >>>
> >>>Probelm is, reducing the SPIN_THRESHOLD is resulting in excess halt
> >>>exits in under-commits and increasing ple_window may be sometimes
> >>>counter productive as it affects other busy-wait constructs such as
> >>>flush_tlb AFAIK.
> >>>So if we could have had a dynamically changing SPIN_THRESHOLD too, that
> >>>would be nice.
> >>>
> >>
> >>Gleb, Andrew,
> >>I tested with the global ple window change (similar to what I posted
> >>here https://lkml.org/lkml/2012/11/11/14 ),
> >This does not look global. It changes PLE per vcpu.
> 
> Okay. Got it. I was thinking it would change the global value. But IIRC
>  It is changing global sysfs value and per vcpu ple_window.
> Sorry. I missed this part yesterday.
> 
Yes, it changes sysfs value but this does not affect already created
vcpus.

> >
> >>But did not see good result. May be it is good to go with per VM
> >>ple_window.
> >>
> >>Gleb,
> >>Can you elaborate little more on what you have in mind regarding per
> >>VM ple_window. (maintaining part of it as a per vm variable is clear
> >>to
> >>  me), but is it that we have to load that every time of guest entry?
> >>
> >Only when it changes, shouldn't be to often no?
> 
> Ok. Thinking how to do. read the register and writeback if there need
> to be a change during guest entry?
> 
Why not do it like in the patch you've linked? When value changes write it
to VMCS of the current vcpu.

--
			Gleb.
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