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Date: Mon, 09 Jul 2012 09:55:10 +0200
From: Christian Borntraeger <borntraeger@de.ibm.com>
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To: Raghavendra K T <raghavendra.kt@linux.vnet.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>
In-Reply-To: <20120709062012.24030.37154.sendpatchset@codeblue>
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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