From mboxrd@z Thu Jan 1 00:00:00 1970 From: Raghavendra K T Subject: Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler Date: Tue, 10 Jul 2012 13:57:22 +0530 Message-ID: <4FFBE76A.3090902@linux.vnet.ibm.com> References: <20120709062012.24030.37154.sendpatchset@codeblue> <4FFA8E5E.3070108@de.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4FFA8E5E.3070108@de.ibm.com> Sender: kvm-owner@vger.kernel.org List-Archive: List-Post: To: Christian Borntraeger Cc: "H. Peter Anvin" , Thomas Gleixner , Marcelo Tosatti , Ingo Molnar , Avi Kivity , Rik van Riel , S390 , Carsten Otte , KVM , chegu vinod , "Andrew M. Theurer" , LKML , X86 , Gleb Natapov , linux390@de.ibm.com, Srivatsa Vaddagiri , Joerg Roedel List-ID: 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.