linux-kernel - Re: [PATCH RFC V3 0/3] kvm: Improving directed yield in PLE handler

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Date:	Fri, 13 Jul 2012 00:53:44 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	S390 <linux-s390@...r.kernel.org>, Carsten Otte <cotte@...ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linux390@...ibm.com
CC:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Avi Kivity <avi@...hat.com>,
	Rik van Riel <riel@...hat.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>, X86 <x86@...nel.org>,
	Gleb Natapov <gleb@...hat.com>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Joerg Roedel <joerg.roedel@....com>
Subject: Re: [PATCH RFC V3 0/3] kvm: Improving directed yield in PLE handler

On 07/13/2012 12:47 AM, Raghavendra K T wrote:
> Currently Pause Loop Exit (PLE) handler is doing directed yield to a
> random vcpu on pl-exit. We already have filtering while choosing
> the candidate to yield_to. This change adds more checks while choosing
> a candidate to yield_to.
>
> On a large vcpu guests, there is a high probability of
> yielding to the same vcpu who had recently done a pause-loop exit.
> Such a yield can lead to the vcpu spinning again.
>
> The patchset keeps track of the pause loop exit and gives chance to a
> vcpu which has:
>
>   (a) Not done pause loop exit at all (probably he is preempted lock-holder)
>
>   (b) vcpu skipped in last iteration because it did pause loop exit, and
>   probably has become eligible now (next eligible lock holder)
>
> This concept also helps in cpu relax interception cases which use same handler.

The patches are tested on x86 only since I don't have access to s390
machine. Please let me know if changes are ok on s390.

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