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

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Date:	Wed, 11 Jul 2012 13:16:41 +0200
From:	Alexander Graf <agraf@...e.de>
To:	Avi Kivity <avi@...hat.com>
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 11.07.2012, at 13:04, Avi Kivity wrote:

> On 07/11/2012 01:17 PM, Christian Borntraeger wrote:
>> On 11/07/12 11:06, Avi Kivity wrote:
>> [...]
>>>> Almost all s390 kernels use diag9c (directed yield to a given guest cpu) for spinlocks, though.
>>> 
>>> Perhaps x86 should copy this.
>> 
>> See arch/s390/lib/spinlock.c
>> The basic idea is using several heuristics:
>> - loop for a given amount of loops
>> - check if the lock holder is currently scheduled by the hypervisor
>>  (smp_vcpu_scheduled, which uses the sigp sense running instruction)
>>  Dont know if such thing is available for x86. It must be a lot cheaper
>>  than a guest exit to be useful
> 
> We could make it available via shared memory, updated using preempt
> notifiers.  Of course piling on more pv makes this less attractive.
> 
>> - if lock holder is not running and we looped for a while do a directed
>>  yield to that cpu.
>> 
>>> 
>>>> So there is no win here, but there are other cases were diag44 is used, e.g. cpu_relax.
>>>> I have to double check with others, if these cases are critical, but for now, it seems 
>>>> that your dummy implementation  for s390 is just fine. After all it is a no-op until 
>>>> we implement something.
>>> 
>>> Does the data structure make sense for you?  If so we can move it to
>>> common code (and manage it in kvm_vcpu_on_spin()).  We can guard it with
>>> CONFIG_KVM_HAVE_CPU_RELAX_INTERCEPT or something, so other archs don't
>>> have to pay anything.
>> 
>> Ignoring the name,
> 
> What name would you suggest?
> 
>> 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?

> and the way it implements cpu_relax() by adjusting hw thread priority,

Yeah, I don't think we can intercept relaxing. It's basically a nop-like instruction that gives hardware hints on its current priorities.
That said, we can always add PV code.


Alex

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