linux-kernel - Re: Default governor regardless of cpuidle driver

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Message-ID: <CAJZ5v0ixpAgdbSfLG=ZAKyF1K35XbeMBZE76+wqoqKBCLy_RLQ@mail.gmail.com>
Date:   Mon, 2 Sep 2019 23:58:46 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Joao Martins <joao.m.martins@...cle.com>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        kvm-devel <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: Default governor regardless of cpuidle driver

On Fri, Aug 30, 2019 at 1:09 PM Joao Martins <joao.m.martins@...cle.com> wrote:
>
> On 8/29/19 10:51 PM, Daniel Lezcano wrote:
> > On 29/08/2019 23:12, Joao Martins wrote:
> >
> > [ ... ]
> >
> >>>> Say you wanted to have a kvm specific config, you would still see the same
> >>>> problem if you happen to compile intel_idle together with haltpoll
> >>>> driver+governor.
> >>>
> >>> Can a guest work with an intel_idle driver?
> >>>
> >> Yes.
> >>
> >> If you use Qemu you would add '-overcommit cpu-pm=on' to try it out. ofc,
> >> assuming you're on a relatively recent Qemu (v3.0+) and a fairly recent kernel
> >> version as host (v4.17+).
> >
> > Ok, thanks for the clarification.
> >
> >>>> Creating two separate configs here, with and without haltpoll
> >>>> for VMs doesn't sound effective for distros.
> >>>
> >>> Agree
> >>>
> >>>> Perhaps decreasing the rating of
> >>>> haltpoll governor, but while a short term fix it wouldn't give much sensible
> >>>> defaults without the one-off runtime switch.
> >
> > The rating has little meaning because each governor fits a specific
> > situation (server, desktop, etc...) and it would probably make sense to
> > remove it and add a default governor in the config file like the cpufreq.
> >
> ICYM, I had attached a patch in the first message of this thread [0] right below
> the scissors mark. It's not based on config file, but it's the same thing you're
> saying (IIUC) but at runtime and thus allowing a driver to state a 'preferred'
> governor to switch to at idle registration -- let me know if you think that
> looks a sensible approach. Note that the intent of that patch follows the
> thinking of leaving all defaults as before haltpoll governor was introduced, but
> once user modloads/uses cpuidle-haltpoll this governor then gets switched on.
>
> [0] https://lore.kernel.org/kvm/c8cf8dcc-76a3-3e15-f514-2cb9df1bbbdc@oracle.com/
>
> I would think a config-based preference on a governor would be good *if* one
> could actually switch idle governors at runtime like you can with cpufreq -- in
> case userspace wants something else other than the default. Right now we can't
> do that unless you toggle 'cpuidle_sysfs_switch', or picking one at boot with
> 'cpuidle.governor='.

FWIW, I've been thinking about getting rid of the cpuidle_sysfs_switch
command line option and always allowing user space to switch cpuidle
governors at run time.  At least I see no reason why that would not
work ATM.