From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754787Ab2GQIaK (ORCPT ); Tue, 17 Jul 2012 04:30:10 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53394 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754625Ab2GQIaG (ORCPT ); Tue, 17 Jul 2012 04:30:06 -0400 Message-ID: <50052272.5020803@redhat.com> Date: Tue, 17 Jul 2012 11:29:38 +0300 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120615 Thunderbird/13.0.1 MIME-Version: 1.0 To: Rik van Riel CC: Raghavendra K T , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , Marcelo Tosatti , Srikar , S390 , Carsten Otte , Christian Borntraeger , KVM , chegu vinod , "Andrew M. Theurer" , LKML , X86 , Gleb Natapov , linux390@de.ibm.com, Srivatsa Vaddagiri , Joerg Roedel Subject: Re: [PATCH RFC V4 3/3] kvm: Choose better candidate for directed yield References: <20120716082445.23477.15128.sendpatchset@codeblue.in.ibm.com> <20120716082529.23477.91096.sendpatchset@codeblue.in.ibm.com> <5003E7ED.2030701@redhat.com> <50043CF3.8020903@redhat.com> In-Reply-To: <50043CF3.8020903@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/16/2012 07:10 PM, Rik van Riel wrote: > On 07/16/2012 06:07 AM, Avi Kivity wrote: > >>> +{ >>> + bool eligible; >>> + >>> + eligible = !vcpu->ple.cpu_relax_intercepted || >>> + (vcpu->ple.cpu_relax_intercepted&& >>> + vcpu->ple.dy_eligible); >>> + >>> + if (vcpu->ple.cpu_relax_intercepted) >>> + vcpu->ple.dy_eligible = !vcpu->ple.dy_eligible; >> >> Probably should assign 'true', since the previous value is essentially >> random. > > I suspect the intended purpose of this conditional is to > flip the eligibility of a vcpu for being selected as a > direct yield target. > > In other words, that bit of the code is correct. If vcpu A is in a long spin loop and is preempted away, and vcpu B dips several times in kvm_vcpu_on_spin(), then it will act as intended. But if vcpu A is spinning for x% of its time and processing on the other, then vcpu B will flip its dy_eligible for those x%, and not flip it when it's processing. I don't understand how this is useful. I guess this is an attempt to impose fairness on yielding, and it makes sense to do this, but I don't know if this is the best way to achieve it. -- error compiling committee.c: too many arguments to function