linux-kernel - Re: ...\n

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Message-ID: <Ypc9G3nTkib1y9X4@worktop.programming.kicks-ass.net>
Date:   Wed, 1 Jun 2022 12:19:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Durrant, Paul" <pdurrant@...zon.co.uk>,
        "Allister, Jack" <jalliste@...zon.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "diapop@...zon.co.uk" <diapop@...zon.co.uk>,
        "hpa@...or.com" <hpa@...or.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "metikaya@...zon.co.uk" <metikaya@...zon.co.uk>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: ...\n

On Wed, Jun 01, 2022 at 10:59:17AM +0200, Paolo Bonzini wrote:
> On 6/1/22 09:57, Vitaly Kuznetsov wrote:
> > > > I'll bite... What's ludicrous about wanting to run a guest at a lower CPU freq to minimize observable change in whatever workload it is running?
> > > Well, the right API is cpufreq, there's no need to make it a KVM
> > > functionality.
> > KVM may probably use the cpufreq API to run each vCPU at the desired
> > frequency: I don't quite see how this can be done with a VMM today when
> > it's not a 1-vCPU-per-1-pCPU setup.
> 
> True, but then there's also a policy issue, in that KVM shouldn't be allowed
> to *bump* the frequency if userspace would ordinarily not have access to the
> cpufreq files in sysfs.

So, when using schedutil (which requires intel_pstate in passive mode),
then there's the option to use per-task uclamps which are somewhat
complicated but also affect cpufreq.