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Date: Tue, 10 Jul 2012 13:57:22 +0530
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Organization: IBM
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To: Christian Borntraeger <borntraeger@de.ibm.com>
CC: "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
        Marcelo Tosatti <mtosatti@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Avi Kivity <avi@redhat.com>, Rik van Riel <riel@redhat.com>,
        S390 <linux-s390@vger.kernel.org>, Carsten Otte <cotte@de.ibm.com>,
        KVM <kvm@vger.kernel.org>, chegu vinod <chegu_vinod@hp.com>,
        "Andrew M. Theurer" <habanero@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 <x86@kernel.org>,
        Gleb Natapov <gleb@redhat.com>, linux390@de.ibm.com,
        Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>,
        Joerg Roedel <joerg.roedel@amd.com>
Subject: Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler
References: <20120709062012.24030.37154.sendpatchset@codeblue> <4FFA8E5E.3070108@de.ibm.com>
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On 07/09/2012 01:25 PM, Christian Borntraeger wrote:
> 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 for the review. Nice to know that, patch has potential to help 
s390 also.