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

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Message-ID: <4FFA8E5E.3070108@de.ibm.com>
Date:	Mon, 09 Jul 2012 09:55:10 +0200
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
CC:	"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>,
	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>
Subject: Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

On 09/07/12 08:20, Raghavendra K T wrote:
> Currently Pause Looop Exit (PLE) handler is doing directed yield to a
> random VCPU on PL exit. Though we already have filtering while choosing
> the candidate to yield_to, we can do better.
> 
> Problem is, for large vcpu guests, we have more probability of yielding
> to a bad vcpu. We are not able to prevent directed yield to same guy who
> has done PL exit recently, who perhaps spins again and wastes CPU.
> 
> Fix that by keeping track of who has done PL exit. So The Algorithm in series
> give chance to a VCPU which has:


We could do the same for s390. The appropriate exit would be diag44 (yield to hypervisor).

Almost all s390 kernels use diag9c (directed yield to a given guest cpu) for spinlocks, though.
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.

Thanks

Christian

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