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

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Message-ID: <4FFD68AA.1000607@linux.vnet.ibm.com>
Date:	Wed, 11 Jul 2012 17:21:06 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
CC:	Avi Kivity <avi@...hat.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>
Subject: Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

On 07/11/2012 03:47 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

Unfortunately we do not have information on lock-holder.

> - 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, yes the data structure itself seems based on the algorithm
> and not on arch specific things. That should work.

Ok. can you please elaborate, on the flow.

  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.
>
> So this would probably improve guests that uses cpu_relax, for example
> stop_machine_run. I have no measurements, though.
>
> Christian
>

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