linux-kernel - Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler

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Date:	Thu, 27 Sep 2012 10:50:30 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jiannan Ouyang <ouyang@...pitt.edu>
CC:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, chegu vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Gleb Natapov <gleb@...hat.com>
Subject: Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE
 handler

On 09/25/2012 04:43 PM, Jiannan Ouyang wrote:
> I've actually implemented this preempted_bitmap idea. 

Interesting, please share the code if you can.

> However, I'm doing this to expose this information to the guest, so the
> guest is able to know if the lock holder is preempted or not before
> spining. Right now, I'm doing experiment to show that this idea works.
> 
> I'm wondering what do you guys think of the relationship between the
> pv_ticketlock approach and PLE handler approach. Are we going to adopt
> PLE instead of the pv ticketlock, and why?

Right now we're searching for the best solution.  The tradeoffs are more
or less:

PLE:
- works for unmodified / non-Linux guests
- works for all types of spins (e.g. smp_call_function*())
- utilizes an existing hardware interface (PAUSE instruction) so likely
more robust compared to a software interface

PV:
- has more information, so it can perform better

Given these tradeoffs, if we can get PLE to work for moderate amounts of
overcommit then I'll prefer it (even if it slightly underperforms PV).
If we are unable to make it work well, then we'll have to add PV.

-- 
error compiling committee.c: too many arguments to function
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