linux-kernel - Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

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Message-ID: <4FFD621D.5090607@redhat.com>
Date:	Wed, 11 Jul 2012 14:23:09 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Alexander Graf <agraf@...e.de>
CC:	Christian Borntraeger <borntraeger@...ibm.com>,
	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>, Rik van Riel <riel@...hat.com>,
	S390 <linux-s390@...r.kernel.org>,
	Carsten Otte <cotte@...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>, X86 <x86@...nel.org>,
	Gleb Natapov <gleb@...hat.com>, linux390@...ibm.com,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Joerg Roedel <joerg.roedel@....com>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

On 07/11/2012 02:16 PM, Alexander Graf wrote:
>> 
>>> yes the data structure itself seems based on the algorithm
>>> and not on arch specific things. That should work. If we move that to common 
>>> code then s390 will use that scheme automatically for the cases were we call 
>>> kvm_vcpu_on_spin(). All others archs as well.
>> 
>> ARM doesn't have an instruction for cpu_relax(), so it can't intercept
>> it.  Given ppc's dislike of overcommit,
> 
> What dislike of overcommit?

I understood ppc virtualization is more of the partitioning sort.
Perhaps I misunderstood it.  But the reliance on device assignment, the
restrictions on scheduling, etc. all point to it.

> 
>> and the way it implements cpu_relax() by adjusting hw thread priority,
> 
> Yeah, I don't think we can intercept relaxing.

... and the lack of ability to intercept cpu_relax() ...

> It's basically a nop-like instruction that gives hardware hints on its current priorities.

That's what x88 PAUSE does.  But we can intercept it (and not just any
execution - we can restrict intercept to tight loops executed more than
a specific number of times).

> That said, we can always add PV code.

Sure, but that's defeated by advancements like self-tuning PLE exits.
It's hard to get this right.

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


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