From mboxrd@z Thu Jan 1 00:00:00 1970 From: Raghavendra K T Subject: Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler Date: Mon, 24 Sep 2012 17:22:40 +0530 Message-ID: <50604988.2030506@linux.vnet.ibm.com> References: <20120921115942.27611.67488.sendpatchset@codeblue> <1348486479.11847.46.camel@twins> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: "H. Peter Anvin" , Marcelo Tosatti , Ingo Molnar , Avi Kivity , Rik van Riel , Srikar , "Nikunj A. Dadhania" , KVM , Jiannan Ouyang , chegu vinod , "Andrew M. Theurer" , LKML , Srivatsa Vaddagiri , Gleb Natapov , Andrew Jones To: Peter Zijlstra Return-path: In-Reply-To: <1348486479.11847.46.camel@twins> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 09/24/2012 05:04 PM, Peter Zijlstra wrote: > On Fri, 2012-09-21 at 17:29 +0530, Raghavendra K T wrote: >> In some special scenarios like #vcpu<= #pcpu, PLE handler may >> prove very costly, because there is no need to iterate over vcpus >> and do unsuccessful yield_to burning CPU. > > What's the costly thing? The vm-exit, the yield (which should be a nop > if its the only task there) or something else entirely? > Both vmexit and yield_to() actually, because unsuccessful yield_to() overall is costly in PLE handler. This is because when we have large guests, say 32/16 vcpus, and one vcpu is holding lock, rest of the vcpus waiting for the lock, when they do PL-exit, each of the vcpu try to iterate over rest of vcpu list in the VM and try to do directed yield (unsuccessful). (O(n^2) tries). this results is fairly high amount of cpu burning and double run queue lock contention. (if they were spinning probably lock progress would have been faster). As Avi/Chegu Vinod had felt it is better to avoid vmexit itself, which seems little complex to achieve currently.