linux-kernel - Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler

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Message-ID: <20120926133905.GA8842@turtle.usersys.redhat.com>
Date:	Wed, 26 Sep 2012 15:39:05 +0200
From:	Andrew Jones <drjones@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Avi Kivity <avi@...hat.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Rik van Riel <riel@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>,
	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 0/2] kvm: Improving undercommit,overcommit scenarios
 in PLE handler

On Wed, Sep 26, 2012 at 03:26:11PM +0200, Peter Zijlstra wrote:
> On Wed, 2012-09-26 at 15:20 +0200, Andrew Jones wrote:
> > Wouldn't a clean solution be to promote a task's scheduler
> > class to the spinner class when we PLE (or come from some special
> > syscall
> > for userspace spinlocks?)? 
> 
> Userspace spinlocks are typically employed to avoid syscalls..

I'm guessing there could be a slow path - spin N times and then give
up and yield.

> 
> > That class would be higher priority than the
> > fair class and would schedule in FIFO order, but it would only run its
> > tasks for short periods before switching. 
> 
> Since lock hold times aren't limited, esp. for things like userspace
> 'spin' locks, you've got a very good denial of service / opportunity for
> abuse right there.

Maybe add some throttling to avoid overuse/maliciousness?

> 
> 
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