From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751386AbdBXPfJ (ORCPT ); Fri, 24 Feb 2017 10:35:09 -0500 Received: from mx1.redhat.com ([209.132.183.28]:54054 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751290AbdBXPfC (ORCPT ); Fri, 24 Feb 2017 10:35:02 -0500 Subject: Re: [patch 0/3] KVM CPU frequency change hypercalls To: Marcelo Tosatti References: <20170202174755.946578704@redhat.com> <20170203164349.GA5582@potion> <20170203181405.GA1869@amt.cnet> <20170203190930.GD15128@potion> <5db0e1a1-2eaf-fe77-3e15-7b2ef842d255@redhat.com> <20170223231923.GB23631@amt.cnet> <5310a7a1-3c3b-7b19-4f2d-6f7919c7b560@redhat.com> <20170224114958.GA28618@amt.cnet> <70575c62-b9c5-7100-193a-5f4b8bed258c@redhat.com> <20170224130458.GA31610@amt.cnet> Cc: Radim Krcmar , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, "Rafael J. Wysocki" , Viresh Kumar From: Paolo Bonzini Message-ID: <2c739f77-22c6-c14c-c279-f2dd2445b2df@redhat.com> Date: Fri, 24 Feb 2017 16:34:52 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0 MIME-Version: 1.0 In-Reply-To: <20170224130458.GA31610@amt.cnet> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Fri, 24 Feb 2017 15:34:57 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 24/02/2017 14:04, Marcelo Tosatti wrote: >>>>> Whats the current usecase, or forseeable future usecase, for save/restore >>>>> across preemption again? (which would validate the broken by design >>>>> claim). >>>> Stop a guest that is using cpufreq, start a guest that is not using it. >>>> The second guest's performance now depends on the state that the first >>>> guest left in cpufreq. >>> Nothing forbids the host to implement switching with the >>> current hypercall interface: all you need is a scheduler >>> hook. >> Can it be done in vcpu_load/vcpu_put? But you still would have two >> components (KVM and sysfs) potentially fighting over the frequency, and >> that's still a bit ugly. > > Change the frequency at vcpu_load/vcpu_put? Yes: call into > cpufreq-userspace. But there is no notion of "per-task frequency" on the > Linux kernel (which was the starting point of this subthread). There isn't, but this patchset is providing a direct path from a task to cpufreq-userspace. This is as close as you can get to a per-task frequency. > But if you configure all CPUs in the system as cpufreq-userspace, > then some other (userspace program) has to decide the frequency > for the other CPUs. > > Which agent would do that and why? Thats why i initially said "whats the > usecase". You could just pin them at the highest non-TurboBoost frequency until a guest runs. That's assuming that they are idle and, because of isol_cpus/nohz_full, they would be almost always in deep C state anyway. Paolo